Reverted mongoid generator for to use config/database.rb instead of
config/database.yml.

However, I left the indications on how to use that on the generated
config/database.rb.

Also implemented ENV['MONGO_URI'] as @j15e's suggested. It will default
to it if set.
Merge pull request #1108 from padrino/revert-mongoid-to-use-database-rb

Reverted mongoid generator to use config/database.rb instead of yml file.