EltwiseProductLayer -> EltwiseLayer for generality

Reproduce elementwise product layer in more generality.
Add elementwise operation parameter.
Prepare for elementwise sum operation choice.
add caffe_gpu_add() and caffe_gpu_sub()
Elementwise layer learns summation
add EltwiseLayer docstring
fix layer name in logging
make sum the default eltwise operation
weight elementwise sum with per-blob coefficients
commment, lint
Merge pull request #433 from shelhamer/eltwise

Elementwise layer takes sum or product; caffe_gpu_{add,sub}