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. 
Wait, which paper are you taking these equations from?
In http://www.cs.toronto.edu/~hinton/absps/momentum.pdf, equations (3) and (4) do not look like yours, especially because their gradient is taken at `\theta_t + \mu_t v_t`, not at `\theta_t`.