support for unpacking megolm keys

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

lib/crypto.js

@@ -700,6 +700,26 @@ Crypto.prototype.decryptEvent = function(event) {
     return alg.decryptEvent(event);
 };
 
+
+/**
+ * Handle a key event
+ *
+ * @param {module:modules/event~MatrixEvent} event key event
+ */
+Crypto.prototype.onRoomKeyEvent = function(event) {
+    var content = event.getContent();
+    var AlgClass = algorithms.DECRYPTION_CLASSES[content.algorithm];
+    if (!AlgClass) {
+        throw new algorithms.DecryptionError(
+            "Unable to handle keys for " + content.algorithm
+        );
+    }
+    var alg = new AlgClass({
+        olmDevice: this._olmDevice,
+    });
+    alg.onRoomKeyEvent(event);
+};
+
 /**
  * @see module:crypto-algorithms/base.DecryptionError
  */