Make date and datetime sortable when using Enumberable#sort_by

Fixes issue: comparison of Icalendar::Values::Date with Icalendar::Values::Date failed
Adds 3 tests and fixtures.

Tests:
 - sorting 2 date based events
 - sorting 2 time based events
 - sorting 2 events with disparate types
Use date and date_time #value instead of forcing them to date and date_time

This lets us compare a Date with a DateTime.
Sort dates on string representation to mix date and datetime
Merge branch 'markrickert-sortable-dates'

* markrickert-sortable-dates:
  Sort dates on string representation to mix date and datetime
  Use date and date_time #value instead of forcing them to date and date_time
  Adds 3 tests and fixtures.
  Make date and datetime sortable when using Enumberable#sort_by