Set up a cucumber test suite covering the commands in the quick start guide

I've been wanting to automate this for a while now. Testing that series of
commands is really tedious, but on the other hand, shipping a version of the gem
that breaks on usage covered in the introductory documentation would be pretty
humiliatingly bad. So this should hopefully allow me to spend less time on the
dullest form of testing, without any sacrifice in terms of quality.