Always parse months as 1-indexed

This completes ce813fd01, fixes #128 fully.

I suspect you may not like this merge, as you deliberately have added this zero-indexed parsing, but please take a look at my [issue Fiddle](http://jsfiddle.net/vZ6af/). It results in a very strange behaviour that cost me some time to debug.

My fix may be wrong and we should do differently, but my expectation in the Fiddle was absolutely grounded, and I will argue for the usability of the library to change the behavior.