support for unpacking megolm keys

This is incredibly hacky at the moment, pending the arrival of ephemeral
events, but it kinda works.

lib/crypto-algorithms/base.js

@@ -93,12 +93,16 @@ EncryptionAlgorithm.prototype.initRoomEncryption = function(roomMembers) {
  * base type for decryption implementations
  *
  * @constructor
+ * @alias module:crypto-algorithms/base.DecryptionAlgorithm
+ *
  * @param {object} params parameters
  * @param {module:OlmDevice} params.olmDevice olm.js wrapper
  */
-module.exports.DecryptionAlgorithm = function(params) {
+var DecryptionAlgorithm = function(params) {
     this._olmDevice = params.olmDevice;
 };
+/** */
+module.exports.DecryptionAlgorithm = DecryptionAlgorithm;
 
 /**
  * Decrypt an event
@@ -114,6 +118,17 @@ module.exports.DecryptionAlgorithm = function(params) {
  *   problem decrypting the event
  */
 
+/**
+ * Handle a key event
+ *
+ * @method module:crypto-algorithms/base.DecryptionAlgorithm#onRoomKeyEvent
+ *
+ * @param {module:modules/event~MatrixEvent} event key event
+ */
+DecryptionAlgorithm.prototype.onRoomKeyEvent = function(params) {
+    // ignore by default
+};
+
 /**
  * Exception thrown when decryption fails
  *