Add command-line component generator

Adds a generator that scaffolds up a complete, but minimal, component
using JSX and a propTypes hash.

Run `rails generate react:component --help` for usage notes.
Document the command line component generator
Merge remote-tracking branch 'upstream/master' into component-generator
Changes to work with React 0.12 and follow @zpao's recommendations
Fix outdated README notes on the component generator
Merge pull request #96 from xionon/component-generator

Add command-line component generator