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-algorithms/olm.js

@@ -20,6 +20,7 @@ limitations under the License.
  *
  * @module crypto-algorithms/olm
  */
+var q = require('q');
 
 var utils = require("../utils");
 
@@ -60,7 +61,7 @@ OlmEncryption.prototype.initRoomEncryption = function(roomMembers) {
  * @param {string} eventType
  * @param {object} plaintext event content
  *
- * @return {object} new event body
+ * @return {module:client.Promise} Promise which resolves to the new event body
  */
 OlmEncryption.prototype.encryptMessage = function(room, eventType, content) {
     if (!room) {
@@ -128,7 +129,7 @@ OlmEncryption.prototype.encryptMessage = function(room, eventType, content) {
         sender_key: this._olmDevice.deviceCurve25519Key,
         ciphertext: ciphertext
     };
-    return encryptedContent;
+    return q(encryptedContent);
 };
 
 /**