add mxc -> http uri conversion func to client

lib/client.js

@@ -713,6 +713,20 @@ MatrixClient.prototype.getAvatarUrlForMember =
     return null;
 };
 
+/**
+ * Turn an MXC URL into an HTP one
+ * @param {string} mxcUrl The MXC URL
+ * @param {Number} width The desired width of the thumbnail.
+ * @param {Number} height The desired height of the thumbnail.
+ * @param {string} resizeMethod The thumbnail resize method to use, either
+ * "crop" or "scale".
+ * @return {?string} the avatar URL or null.
+ */
+MatrixClient.prototype.mxcUrlToHttp =
+        function(mxcUrl, width, height, resizeMethod) {
+    return this._http.getHttpUriForMxc(mxcUrl, width, height, resizeMethod);
+};
+
 /**
  * @param {module:client.callback} callback Optional.
  * @return {module:client.Promise} Resolves: TODO