Add some tests for the Rails' generators

Add basic tests for the Rails generators provided with Bear. Basically
just create a new Rails application and run most of the available
generators and their options.

Rely on Aruba to check file/folder presence and content and also for
steps related to the command line.

Bear requires Rails as a _development_ dependency, it could work with
Rails 3.1+ but support for these versions is not on the roadmap.
Move each generator to its own directory

Move the layout and structure generator to its own folder to keep it
clean and don't store all templates in the same directory.

Fix on the fly a call to empty_folder -> empty_directory in the
structure generator.
Fix the generator tests

Fix the Cucumber test suite passing the --force option to the rails
generators to avoid time out errors because we won't interact with the
shell script.

Also now generate a whole new application for each scenario.

The step definitions have also been fixed as well to rely on Ruby core
to check file/directory presence.