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.