Allow authentication keys to be passed in memory.

All credits go to Joe Turpin, I'm just reaplying and cleaning his patch:
http://www.libssh2.org/mail/libssh2-devel-archive-2012-01/0015.shtml

* Use an unimplemented error for extracting keys from memory with libgcrypt.

include/libssh2.h

@@ -576,6 +576,16 @@ libssh2_userauth_hostbased_fromfile_ex(LIBSSH2_SESSION *session,
                                         (username),                     \
                                         (unsigned int)strlen(username))
 
+LIBSSH2_API int
+libssh2_userauth_publickey_frommemory(LIBSSH2_SESSION *session,
+                                      const char *username,
+                                      size_t username_len,
+                                      const char *publickeyfiledata,
+                                      size_t publickeyfiledata_len,
+                                      const char *privatekeyfiledata,
+                                      size_t privatekeyfiledata_len,
+                                      const char *passphrase);
+
 /*
  * response_callback is provided with filled by library prompts array,
  * but client must allocate and fill individual responses. Responses