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/timeline-window.js

@@ -41,7 +41,7 @@ var DEFAULT_PAGINATE_LOOP_LIMIT = 5;
  * Construct a TimelineWindow.
  *
  * <p>This abstracts the separate timelines in a Matrix {@link
- * module:models/room~Room|Room} into a single iterable thing. It keeps track of
+ * module:models/room|Room} into a single iterable thing. It keeps track of
  * the start and endpoints of the window, which can be advanced with the help
  * of pagination requests.
  *