add SILENCE layer -- takes one or more inputs and produces no output

This is useful for suppressing undesired outputs.

Conflicts:
	include/caffe/common_layers.hpp
	src/caffe/proto/caffe.proto
add SILENCE layer -- takes one or more inputs and produces no output

This is useful for suppressing undesired outputs.

Conflicts:
	include/caffe/common_layers.hpp
	src/caffe/proto/caffe.proto