Add browser tests for BOSH functionality

Test the browserified version of `node-xmpp-client` BOSH package.

Note:

* We still need to use `preferred: 'PLAIN'` this needs to be fixed
Well, the registration flag just checks if something is 'truthy' rather than strict true/false boolean so I'm not sure what this would achieve. If its a blocker for you I'll happily add one.