Updating for Python 2.7.6

I tested this on my Mavericks machine and it worked. It solved the issues I was having installing some Python based homebrew packages using Boxen, but I'm not saying it's perfect. Please test and update. I haven't tested any of the other features (requirements, virtualenv, etc.), just did `include python` and got Python 2.7.6 in the right place, with pip as well.

- Replace brews/python.rb with current stock homebrew formula
- Replace $BOXEN_HOME/homebrew assumptions with $HOMEBREW_ROOT
- Bump to 2.7.6-boxen3
- Remove file resources (managed in brew formula)
- Removed pip empty parameter validation (Puppet already requires setting)
- Update virtualenvwrapper to 4.2

When ready to merge, I'd vote a version bump to 1.4.0.