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")
Merge branch 'generators' into develop