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
This is a generic bosh test with node-xmpp-client. Have I got this right? I was confused to see the same tests twice. We may move this file to test/bosh.js...