Allow LaTeX display for NMatrix

This commit adds support for printing NMatrices via LaTeX. Not very much
to explain!

This is how it looks like:

![screen shot 2014-04-27 at 9 05 53 pm](https://cloud.githubusercontent.com/assets/615227/2812695/51cbb380-ce69-11e3-80bc-f298fde41833.png)