gitian: Add OSX build descriptors

This is a set of descriptors for building an OSX DMG without the need for OSX. The descriptors are quite large and unruly, but this seemed like the most reasonable first step towards releasing builds created this way.

These have been ported to monolithic descriptors from the work done in my repo here:
https://github.com/theuni/osx-cross-depends/

I've included a Readme that attempts to cover the most obvious questions. After some major changes in the last week, the process is actually rather clean now and contains very few nasty hacks. The exception to that is cctools, which is... gross. See the readme for details.

Unfortunately my macbook completely died yesterday (new one is on the way), so I'm unable to help with testing until next week. Though I've made no major functional changes since then (worked fine as of yesterday), so I'm confident that the resulting installer is in good shape. Much testing is needed, of course.

There are still plenty of warts in the process, and I have plans for addressing them. As discussed with @laanwj, this is intended to be only the first step.