Adds RailsGenerator to simplify setup in a Rails app.

I thought you may find this feature useful for your gem.

In order to ease installation, you can use a Rails Generator now:

`$ rails generate jquery:datatables:install`
`$ rails generate jquery:datatables:install bootstrap2`
`$ rails generate jquery:datatables:install bootstrap3`
`$ rails generate jquery:datatables:install foundation`
`$ rails generate jquery:datatables:install responsive`