Allow tz-aware inputs in Holiday.dates
closes #9825

Previously, passing tz-aware inputs to Holiday.dates would always result in an error because the the input start_date was converted in a way that destroyed timezone information.  This PR makes that conversion correctly preserve tz info, and converts several of the existing tests to run with both tz-aware and tz-naive inputs.

Noticed while working on a new trading calendar implementation here: https://github.com/quantopian/zipline/pull/556.
