Hipchat back-end overhaul/MUC implementation

This PR mainly implements the MUC management features as described in #283. In the process of implementing this, I did however choose to scrap the limited html support that was there, my reasoning for which can be found in the [commit message](https://github.com/zoni/err/commit/aef572ab790e0510d247499087039b8f8bf185fd).