* MAINT: Use TradingCalendar objects for bundles
Instead of trading days, opens, and closes, register now takes a
TradingCalendar object, along with a start_session and end_session. The
ingest function is now passed these values instead as well.
* Accept calendar name in addition to the actual object
* Updates bundles documentation for changes
* Fix typo in docs
* Use class formatting
* Force start_session and end_session within the bounds of the calendar
* Use UTC timestamps in test_core
* Document Trading Calendar API in appendix.rst