Create a installation generator

[Heavily based on the ember.js bootstrap generator]

Running rails generate react:install will append the basic require directives
to application.js

Possible avenues for upgrades:

  - allow for files with arbitrary names ("mycomponents.js" instead of
    "application.js")

  - allow for files with different extensions ("application.js.coffee"
    instead of "application.js")
Allow "upgrades" from existing react apps

react_ujs and the autorequiring of components is new in master. The
install generator now accounts for some application.js files already
having react installed.
Merge remote-tracking branch 'upstream/master' into generators
Minimum necessary README additions neccessary
Update explanation of the installation generator
Update README.md
Merge pull request #64 from xionon/generators

[feedback requested] Create a installation generator