- Support "Time-Range" type objects by allowing optional start and end time parameters such as:

   by_star_field :start_time, :end_time

(This is fully backwards compatible)
- Restructure classes/dirs to be more agnostic toward AR vs. Mongoid
- Consolidate shared methods between AR and Mongoid into a ByStar::Base object
- ActiveRecord should be dev dependency. ActiveSupport should be prod dependency
- Use ActiveSupport::Concern