Refine pruning logic (#222)

Add finer control over the pruning logic, to accommodate more pruning
use-cases.
The full description of the new logic is available in the updated [documentation
of the CompressionScheduler](https://nervanasystems.github.io/distiller/schedule.html#pruning-fine-control), which is also part of this PR.

In this PR:

* Added a new callback to the CompressionScheduler:
compression_scheduler.before_parameter_optimization which is invoked
after the gradients are are computed, but before the weights are updated
by the optimizer.

* We provide an option to mask the gradients, before the weights are updated by the optimizer. 
We register to the parameter backward hook in order to mask the gradients.
This gives us finer control over the parameter updates.

* Added several DropFilter schedules.
DropFilter is a method to regularize networks, and it can also be
used to "prepare" a network for permanent filter pruning.

*Added documentation of pruning fine-control