Add Rails as development dependency, example app

The example application will allow to test the gem, it was generated
by `rails new example --skip-test-unit` to be used with RSpec.
Add RVM-related documentation to example/README

Add reference to the example app README in the gem README
Add RSpec, Factory Girl to example app
Add Custom Rails Generators to the exapmle app
Add Cucumber, Aruba as development dependencies

They'll allow for automated RSpec output testing.
Add gem smoke test using the example app

I believed I could use the .rvmrc feature to switch gemsets while
performing tests with Cucumber, but that doesn't seem to be possible,
or al least I didn't succeed at getting it working. Despite directory
changes and gemset switches, Bundler insists in using the top-level
Gemfile to install, even from the `example` directory.

To work around this issue, I've added the examplle app required gems as
development dependencies. It's not elegant, but it allows to perform
automated testing.
Merge branch 'chore-add-example-app'