Adds time zone aware data grouping method to ActiveRecord.
If you want to group a large set of database rows by date using a timestamp
as reference, the optimal way is to do it right in the database.
When doing it though there are some issues with time zones that #group_date solves.
For example, if a UTC timestamp is set to 2013-12-01 01:00 and you're in Brazil (= 2013-11-30 22:00) you'd want date() to return 2013-11-30 but database will actually return 2013-12-01 as it doesn't know anything about time zones.