Added an approximation in months.

I thought that it could be great to have an approximation in months as a high days number is harder to understand for people.

Thus, I just added an argument in both distanceOfTimeInWordsFilter and timeAgoInWordsFilter functions.

An example on my project:
![capture decran 2013-08-27 a 10 27 56](https://f.cloud.github.com/assets/748707/1032009/ac33a1ca-0ef2-11e3-9500-08080af12bba.png)