Private JID resolving

Hi,

Here is the patch to fix JID resolving. It is a bit invasive but I tried my best not to be.

I first tried to put the private chat JID in the brain but because of a timing issue (https://github.com/github/hubot/issues/619) I reverted that to a class variable.

Please let me know if there is anything you want me to change.

Note:
* I added a FIXME that I would like to discuss about the brain user name (id) used. I really think it should be a the full group chat JID but I didn't wanted to break existing code.
* Fixed the send method that used "envelope.user.id" which does not exists (envelope.user.name)
* I upgraded mocha to the latest version and fixed the Makefile
* I added doc about this feature

Cheers,

Manuel