Added time zone support

Closes #2 

Added timezone support to the stats screen.  Until the shared iOS repo has a lightweight data model to support passing values around, a parameter was added to the view controller, stats service, and stats remote service init methods.  This time zone will need to be set when creating the services but it is allowed to be nil on the view controller (defaults to system timezone).

Opted to not keep the view controller's original init method signature to force consumers of the library to update their code to remain functional.  Not following site timezone is broken behavior.

The time zone is really only used to calculate the date parameters being passed to the remote service.  The scenario that was broken is when the device timezone is very different from the site timezone - the dates could be in the "future" (or past) and today would show no stats.  eg: device is in Tokyo time and site is in central US time - the days change ahead many hours before they do in the US and today would be inaccurate.
It's great to see Assertions around!