Implemented batch filter

This PR implements the [batch filter](http://twig.sensiolabs.org/doc/filters/batch.html) and some basic tests. 

This is my first PR against Twig.js, let me know if it's missing anything.

I put ``chunkArray`` in Twig.lib, however, it could be a nested function inside of ``batch`` if you prefer. I checked  Twig and ``array_chunk`` is only used twice in the library including it's use in the ``batch`` filter.