Added MSRAFiller, an Xavier-like filler designed for use with ReLUs

Added MSRAFiller, an Xavier-like filler designed for use with ReLUs

...instead of tanh. Based on paper: He et al, "Delving Deep into
Rectifiers: Surpassing Human-Level Performance on ImageNet
Classification," 2015.

- add VarianceNorm option to FillerParameters which allows one to
  normalize by fan_in, fan_out or their average.
- update XavierFiller to use the VarianceNorm option (default behavior
  unchanged).
- add tests for MSRAFiller and XavierFiller.