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
Do we really need to use git submodules? Wouldn't it be better if we load it as a bower component? eg. via https://github.com/yatskevich/grunt-bower-task