Code cleanup + Mongoid options pass-thru

Some code cleanup:
- Upgrade to Ruby 1.9 hash syntax
- Remove comment mentioning :default_type option, which does not exist
- Convert string-based class eval syntax to block based

In addition, I've added a feature to pass-thru any valid keys in the options hash to Mongoid (:default, :localize, etc.), with the exception of :type which will always be Array.