Feature/engine support (Issue #11)

Addresses #11 by generating an env.rb file that loads routes for any mounted engine and for the application itself (loads url_helpers for main_app and 'engine_name')

Warning, it's ugly... The thought of this code going into every spinach user's env.rb file grosses me out.

Perhaps it could be moved into the [railties file](https://github.com/codegram/spinach-rails/blob/master/lib/spinach-rails/railtie.rb#L13)...