Changed calendar to start on firstDayOfWeek instead of hardcoding Sunday

[angelolloqui]

Current version of calendar starts in Sunday, and it is hardcoded. In Europe, calendars start on Monday, and actually the firstWeekday indicates the first day of the user's calendar.

The current PR includes code that modifies the way the indexPaths and dates are calculated to use the calendar configuration. Besides, I have added the + (void)setGregorianCalendar:(NSCalendar *)gregorianCalendar; to allow modification of this or other properties (required for Swift)

[AlexeyPoldeo]

Very useful patch, thanks! You need to publish your fork on cocoapods...