Make encryption asynchronous

We're going to need to send out a load of messages to distribute the megolm
keys; as a first step, deal with the asynchronicity this will require.

lib/crypto.js

@@ -600,7 +600,7 @@ Crypto.prototype.isSenderKeyVerified = function(userId, algorithm, sender_key) {
  * @param {object} config The encryption config for the room.
  * @param {string[]} roomMembers userIds of room members to start sessions with
  *
- * @return {Object} A promise that will resolve when encryption is setup.
+ * @return {module:client.Promise} A promise that will resolve when encryption is setup.
  */
 Crypto.prototype.setRoomEncryption = function(roomId, config, roomMembers) {
     // if we already have encryption in this room, we should ignore this event
@@ -727,18 +727,21 @@ Crypto.prototype.isRoomEncrypted = function(roomId) {
  *
  * @param {module:models/room?} room destination room. Null if the destination
  *     is not a room we have seen over the sync pipe.
+ *
+ * @return {module:client.Promise?} Promise which resolves when the event has been
+ *     encrypted, or null if nothing was needed
  */
 Crypto.prototype.encryptEventIfNeeded = function(event, room) {
     if (event.isEncrypted()) {
         // this event has already been encrypted; this happens if the
         // encryption step succeeded, but the send step failed on the first
         // attempt.
-        return;
+        return null;
     }
 
     if (event.getType() !== "m.room.message") {
         // we only encrypt m.room.message
-        return;
+        return null;
     }
 
     var roomId = event.getRoomId();
@@ -755,13 +758,14 @@ Crypto.prototype.encryptEventIfNeeded = function(event, room) {
                 "configuration event."
             );
         }
-        return;
+        return null;
     }
 
-    var encryptedContent = alg.encryptMessage(
+    return alg.encryptMessage(
         room, event.getType(), event.getContent()
-    );
-    event.makeEncrypted("m.room.encrypted", encryptedContent);
+    ).then(function(encryptedContent) {
+        event.makeEncrypted("m.room.encrypted", encryptedContent);
+    });
 };
 
 /**