Cleanup: Breakout defined_fields into:
- define_fields: Defines the database field to store the denorm data
- define_fields_method: Defines the list of fields to denormalize
Rename:
define_fields -> define_mongoid_field
define_fields_method -> define_denorm_attrs
fields_method_name -> denorm_attrs_name
Update CHANGELOG.md
Add Travis.yml and Travis CI badge to README.md