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.
`fixDate` suggests there's something wrong with the date. How about something like this:

```obj-c
- (NSDate *)endDateForPeriodUnit:(StatsPeriodUnit)uint includingDate:(NSDate *)date
```

Or maybe `dateForLastDayInPeriod...`