Fix rounding errors caused by coercing an NSTimeInterval into an NSInteger without rounding. fixes #16

* Rework calculations so that `firstDateInCal` actually corresponds to the first date in the calendar
* Change all calculations of time intervals in days to use `[NSDate daysSinceDate:]`
Merge pull request #17 from GateGuru/bugfix/16-fix-off-by-one-date-errors

Fixes off by one date errors from March 11, 2013