Add database_cleaner gem

Add database_cleaner gem to clean your test database during tests:

http://rubygems.org/gems/database_cleaner

@avdi put together an excellent example on how to configure it and what each setting is doing:

http://devblog.avdi.org/2012/08/31/configuring-database_cleaner-with-rails-rspec-capybara-and-selenium/