Refactor to allow overriding the runs controller.

Easily override in main app now. Example and tests included.
[TAV-330] Install generator.

This copies an example Taverna Player initializer into the main app at
config/initializers/taverna_player.rb ready for user customization.
[TAV-330] Callbacks generator.

This copies some example callback stubs into the main app at lib/callbacks.rb
ready for user customization.
[TAV-330] Controllers generator.

This copies customizable controller stubs into the main app so that the Taverna
Player controllers can be overridden.
[TAV-330] Views generator.

This copies the default views into the main app so that they can be customized.