Add debian packaging script and jenkins buildsteps

This adds the needed stuff to build debian packages via jenkins(and without).

The mk-debian-package.sh is generic and by default uses debuild to create a xbmc deb package for the current distribution and architecture, but can be configured to build for arbitrary dists and archs via pbuilder.

The goal here is to host our own debian APT repository. 
Ubuntu packages are not in scope atm, although it would work the same way.

The buildsteps are a shot in the dark, since I have little to no knowledge of how jenkins really works. I'd appreciate help and feedback there.