Add hstore contrib module.

Per discussion
http://archives.postgresql.org/pgsql-hackers/2006-08/msg01409.php

contrib/hstore/README.hstore

@@ -0,0 +1,175 @@
+Hstore - contrib module for storing (key,value) pairs
+
+[Online version] (http://www.sai.msu.su/~megera/oddmuse/index.cgi?Hstore)
+
+Motivation
+
+Many attributes rarely searched, semistructural data, lazy DBA
+
+Authors
+
+    * Oleg Bartunov <oleg@sai.msu.su>, Moscow, Moscow University, Russia
+    * Teodor Sigaev <teodor@sigaev.ru>, Moscow, Delta-Soft Ltd.,Russia
+
+License
+
+Stable version, included into PostgreSQL distribution, released under BSD license. Development version, available from this site, released under the GNU General Public License, version 2 (June 1991).
+
+Operations
+
+    * hstore -> text - get value , perl analogy $h{key} 
+
+select 'a=>q, b=>g'->'a';
+  ?
+------
+  q
+
+    * hstore || hstore - concatenation, perl analogy %a=( %b, %c ); 
+
+regression=# select 'a=>b'::hstore || 'c=>d'::hstore;
+      ?column?      
+--------------------
+ "a"=>"b", "c"=>"d"
+(1 row)
+
+but, notice
+
+regression=# select 'a=>b'::hstore || 'a=>d'::hstore;
+ ?column? 
+----------
+ "a"=>"d"
+(1 row)
+
+    * text => text - creates hstore type from two text strings 
+
+select 'a'=>'b';
+  ?column?
+----------
+  "a"=>"b"
+
+    * hstore @ hstore - contains operation, check if left operand contains right. 
+
+regression=# select 'a=>b, b=>1, c=>NULL'::hstore @ 'a=>c';
+ ?column? 
+----------
+ f
+(1 row)
+
+regression=# select 'a=>b, b=>1, c=>NULL'::hstore @ 'b=>1';
+ ?column? 
+----------
+ t
+(1 row)
+
+    * hstore ~ hstore - contained operation, check if left operand is contained in right
+
+Functions
+
+    * akeys(hstore) - returns all keys from hstore as array 
+
+regression=# select akeys('a=>1,b=>2');
+ akeys 
+-------
+ {a,b}
+
+    * skeys(hstore) - returns all keys from hstore as strings 
+
+regression=# select skeys('a=>1,b=>2');
+ skeys 
+-------
+ a
+ b
+
+    * avals(hstore) - returns all values from hstore as array 
+
+regression=# select avals('a=>1,b=>2');
+ avals 
+-------
+ {1,2}
+
+    * svals(hstore) - returns all values from hstore as strings 
+
+regression=# select svals('a=>1,b=>2');
+ svals 
+-------
+ 1
+ 2
+
+    * delete (hstore,text) - delete (key,value) from hstore if key matches argument. 
+
+regression=# select delete('a=>1,b=>2','b');
+  delete  
+----------
+ "a"=>"1"
+
+    * each(hstore) return (key, value) pairs 
+
+regression=# select * from each('a=>1,b=>2');
+ key | value 
+-----+-------
+ a   | 1
+ b   | 2
+
+    * isexists (hstore,text) - returns 'true if key is exists in hstore and false otherwise. 
+
+regression=# select isexists('a=>1','a');
+ isexists 
+----------
+ t
+
+    * isdefined (hstore,text) - returns true if key is exists in hstore and its value is not NULL. 
+
+regression=# select isdefined('a=>NULL','a');
+ isdefined 
+-----------
+ f
+
+Indices
+
+Module provides index support for '@' and '~' operations.
+
+create index hidx on testhstore using gist(h);
+
+Note
+
+Use parenthesis in select below, because priority of 'is' is higher than that of '->'
+
+select id from entrants where (info->'education_period') is not null;
+
+Examples
+
+    * add key 
+
+update tt set h=h||'c=>3';
+
+    * delete key 
+
+update tt set h=delete(h,'k1');
+
+    * Statistics
+
+hstore type, because of its intrinsic liberality, could contain a lot of different keys. Checking for valid keys is the task of application. Examples below demonstrate several techniques how to check keys statistics.
+
+          o simple example 
+
+select * from each('aaa=>bq, b=>NULL, ""=>1 ');
+
+          o using table 
+
+select (each(h)).key, (each(h)).value into stat from testhstore ;
+
+          o online stat 
+
+select key, count(*) from (select (each(h)).key from testhstore) as stat group by key order by count desc, key;
+    key    | count 
+-----------+-------
+ line      |   883
+ query     |   207
+ pos       |   203
+ node      |   202
+ space     |   197
+ status    |   195
+ public    |   194
+ title     |   190
+ org       |   189
+...................