Add detailed gitian build guide

I'm working on a detailed guide on how to set up a Debian VM for gitian building (w/ LXC).

Hopefully by describing every step in detail, more people will at least try to get into gitian building.

See https://github.com/laanwj/bitcoin/blob/2014_04_debian_gitian_build_doc/doc/gitian-building.md

Comments are welcome!