Add From/To options for Forward and Backward

This PR
* adds six methods to `Net` for starting or stopping the forward or backward passes at particular layers, and
* exposes those methods in pycaffe via keyword args to `Net.forward` and `Net.backward`.

This is useful
* to save computation time when one only wants lower-layer features, without hacking up the prototxt file, and
* to run experiments that involve partial forward, partial backprop, or even changing intermediate data in the middle of a forward or backward pass.