Fix date parameters sent to API

Fixes #78 

Recalculate parameters sent to the API side to pick dates that are at the end of the period.  If today is 12/10/2014 then the parameter sent for fetching this current year's stats should be 12/31/2014.  Also force calendar calculations to Gregorian since that's the only calendar system the backend supports.
Not sure why it's not throwing a warning, but most of the constants are deprecated, see `NSCalendar.h`:

```obj-c
NSYearCalendarUnit NS_CALENDAR_ENUM_DEPRECATED(10_4, 10_10, 2_0, 8_0, "Use NSCalendarUnitYear instead") = NSCalendarUnitYear
```