Full support for Mongoid support + by_quarter + by_calendar_month + fixes for using Date class

1. Added support for Mongoid (adapter for MongoDB). It's basically a monkey-patch file (see: lib/mongoid/by_star.rb) with a small change at the end of lib/by_star.rb to detect inclusion of ActiveRecord and/or Mongoid. I refactored the specs so that the exact same specs can be run by both Mongoid and ActiveRecord seamlessly (core specs now moved to /spec/by_star/generic).

2. Added 'by_quarter' to select by 3-month quarter of year

3. Added 'by_calendar_month', similar to 'by_month' but additionally includes any results which fall in the same week as the first and last of the month. Useful for working with UI calendars which show rows of weeks

4. Some fixes for using Date instead of Time as the input to by_star

5. Added 'between_times' as alias for 'between', since 'between' method name may be used by other gems.

6. Tests for all changes included. Note the only tests which are not passing are those which were not_passing before this commit.

7. Bumped minor version number.

8. Updated Readme.

9. Added support for field-name aliases, which is a Mongo-specific feature (e.g. 'c_at' instead of 'created_at', since Mongo stores column name in each row by design)