Add Support for Calendar Quarters

Hello,

I found your repository as I was looking for ways to calculate pre-defined date ranges for reports.  One of my requirements was to use quarters and I noticed that your library didn't really support that, so I made some changes.  The inspiration for the logic in them comes from http://www.devcurry.com/2009/05/find-first-and-last-day-of-current.html.

Thanks for considering this pull request.

Matthew Belk