Add development tools.
Add 'rake; to Gemfile.
Add code quality rake task.
Add an AbstractMenu interface.

All FlightDeck menus will adhere to a simple interface defined by
AbstractMeny (by inheriting from it). Currently it specifies these:

AbstractMenu#run        # What to do when this entry is selected
AbstractMenu#entry_name # What this menu shows up as in its parent
AbstractMenu#help_text  # What is this menu responsible for

Also adds some tests (introducing a test dir structure) and a rake
task to run tests.