Includes last days of previous month / first days of next month instead of empty boxes

Hi Kyle!
For our needs, I made this change so the calendar grid appear complete.
This is a quick fix which might need some rework, but I thought that may interest others. Feel free to comment and I'll revisit this PR whenever I can.
By the way, the render option is working with [Hogan.js](http://twitter.github.io/hogan.js/) as well.