reformat OS X build instrcs, add 10.8 + Homebrew

I reformatted the entire document to look pretty in both standard text and Markdown. This should make it a lot easier to read.

Also, I incorporated my OS X 10.8 build instructions with Homebrew as the package manager. I originally posted them on the Bitcoin Stackexchange in the question [Trouble compiling bitcoind on OSX with homebrew](http://bitcoin.stackexchange.com/questions/3672/trouble-compiling-bitcoind-on-osx-with-homebrew/5253#5253). I've used these instructions twice now, so I'm fairly certain they'll work.

I do recognize that I need to find a way to get the Makefile to look at Homebrew's Cellar without needing to be patched. WIP on my branch entitled [feature/osx10.8-bitcoind-compilation](https://github.com/colindean/bitcoin/tree/feature/osx10.8-bitcoind-compilation).