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/OlmDevice.js

@@ -15,6 +15,12 @@ limitations under the License.
 */
 "use strict";
 
+/**
+ * olm.js wrapper
+ *
+ * @module OlmDevice
+ */
+
 var Olm = require("olm");
 var utils = require("./utils");
 
@@ -25,6 +31,8 @@ var utils = require("./utils");
  * Accounts and sessions are kept pickled in a sessionStore.
  *
  * @constructor
+ * @alias module:OlmDevice
+ *
  * @param {Object} sessionStore A store to be used for data in end-to-end
  *    crypto
  *