Factor Olm encryption/decryption out to new classes

- to make way for alternative encryption algorithms. We now store an encryption
object for each room, rather than referring to sessionstore on each event.

Also a little light tidying to the jsdocs.

lib/crypto-algorithms/index.js

@@ -0,0 +1,39 @@
+/*
+Copyright 2016 OpenMarket Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+"use strict";
+
+/**
+ * @module crypto-algorithms
+ */
+
+var base = require("./base");
+
+require("./olm");
+
+/**
+ * @see module:crypto-algorithms/base.ENCRYPTION_CLASSES
+ */
+module.exports.ENCRYPTION_CLASSES = base.ENCRYPTION_CLASSES;
+
+/**
+ * @see module:crypto-algorithms/base.DECRYPTION_CLASSES
+ */
+module.exports.DECRYPTION_CLASSES = base.DECRYPTION_CLASSES;
+
+/**
+ * @see module:crypto-algorithms/base.DecryptionError
+ */
+module.exports.DecryptionError = base.DecryptionError;