Add shell of rvm instructions for mac

Prior to this commit, we didn't even have a list of the steps that would
be required for being able to contribute to the website. This commit
adds the shell of the steps required for contributing on a modern-ish
version of Mac OS X.
Add Xcode details

Add Xcode and Command Line Tools details and screenshot.
Nag twitter users to enter an email
Fix a bug that prevented multiple twitter signins
Better test for empty email
Document postgresql requirement for contributing
Flesh out most of the installation instructions
Add in-page links to the overview
Extract OSX instructions into an OSX file
Fix a typo
More info about what to do with questions
Merge pull request #1 from AndrewLorente/add_contribution_requirements_instructions

Document postgresql requirement for contributing
Improvements to OSX_SETUP.md
Instructions for getting set up on Ubuntu

Tested in an Ubuntu 12.04 VM
Instructions for installing Homebrew.

Homebrew is necessary for installing binary dependencies from rvm.
Merge pull request #1 from jeffweiss/add_contribution_requirements_instructions

Add setup instructions for OSX
Really barebones windows setup instructions
Merge pull request #2 from AndrewLorente/add_contribution_requirements_instructions

Add contribution requirements instructions
Merge branch 'master' of github.com:code-scouts/green_mercury
Install a Node runtime on non-OSX platforms