Add calendarWeeks option

Hey,

I needed an option to show the calendar week before each monday, so I implemented it.
Merge if you like :smile: 

![Bildschirmfoto 2013-01-07 um 12 02 27](https://f.cloud.github.com/assets/646693/47158/d075b5cc-58b9-11e2-8f79-8db405d90183.png)

Implementation of the calculation is borrowed from momentjs, I referenced it in the code.

Greetings
1st8