Neural networks

This branch introduces a basic wrapper for OpenCV's multilayer perceptron framework.  While setting training parameters isn't supported yet, the perceptron's kernel and the free parameters of the activation function can be modified.  Other changes include:

* Minor landmark-related transforms.
* EvalRegression will report results only for files that contain the predicted and truth properties.

@bklare please review.
I'm unsure why these changes are showing up in this diff - this stuff is already present in master.