Weight updates only if margin constraint violated.

[npinto]

To decrease communication and speed up convergence, we should have an option (default=True) to only update weights when margin constraints have been violated:

e.g.: Line #66 should move up (to Line #63) and if margin >= 1: continue.

https://github.com/npinto/asgd/blob/master/asgd/naive_asgd.py#L66

[npinto]

Preliminary tests suggest that this will greatly accelerate learning.