Nesterov momentum implementation

Based off of the [simplification suggested by Ian Goodfellow](https://github.com/lisa-lab/pylearn2/issues/677) (@goodfeli) and [the corresponding paper by Yoshua Bengio](http://arxiv.org/pdf/1212.0901v2.pdf).

Still need to add tests to the test suite, but I'm looking for a few more eyes on the code to make sure the basics check out. 
Oh, right! I missed that link, sorry about that.
In that case, yes, it is the right implementation.
You may want to add to the docstring of `nesterov_momentum` that you use the formula in that paper, rather than the one in Sutskever et al.