reformat OS X build instrcs, add 10.8 + Homebrew

* reformatted the entire document to look pretty in both standard text and
  Markdown
* incorporated [my OS X 10.8 build instructions with Homebrew as the package
  manager](http://bitcoin.stackexchange.com/questions/3672/trouble-compiling-bitcoind-on-osx-with-homebrew/5253#5253)
* incorporated @gavinandresen's MacPorts and release build instructions based on
  an earlier revision of this commit
* added makefile patch necessary for Homebrew compilation to `contrib`
Merge pull request #2026 from colindean/doc/reformat-and-update-osx-instructions

reformat OS X build instrcs, add 10.8 + Homebrew