fix browser bugs

Fix two bugs from browser 

The first one cause XMPP.Client undefined in browser. As i mentioned in [issue#196](https://github.com/node-xmpp/node-xmpp/issues/196)

The second , if `faye-websocket` is not installed , the `require('faye-websocket')` in browserify is an Plain Object , and is TRUE , and  `require('faye-websocket').Client` is undefined .