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.
As mentioned on Slack:

> For example, in the Gregorian calendar, n is 7 and Sunday is represented by 1

This might be problematic, since most locales start the week on Monday, and apparently wordpress.com does as well