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
Ensure that the `initialFrame` maintains the initial size used when initializing the calendar. fixes kovpas/PMCalendar#19
Fix `isToday` calculation. fixes kovpas/PMCalendar#20
Merge pull request #21 from GateGuru/bugfix/19-respect-initial-size

Bug fixes for #19 and #20
Merged upstream fixes (except initialSize)