move EngineCart constants into class attributes of EngineCart
add rails application template support
add documentation into generated files
add more inline comments
update readme
use rspec in the test  gem
remove unnecessary test_app_templates from engine_cart
replace the Gemfile.extra approach with just attempting to include the test app's gemfile from the host app.
move engine cart generator to engine_cart:install
Add ENV['ENGINE_CART_RAILS_OPTIONS'] to allow consumers to pass additional options to generate the test app; fixes #7
Merge pull request #8 from cbeer/refactoring

Refactoring