[TOPI] Isolate padding option, improve decl of depthwise/conv2d/pool (#332)

python/tvm/expr.py

python/tvm/make.py

topi/python/topi/__init__.py

topi/python/topi/nn/convolution.py

topi/python/topi/nn/dilate.py

topi/python/topi/nn/pad.py

topi/python/topi/nn/pooling.py

topi/python/topi/nn/softmax.py

topi/python/topi/util.py

topi/recipe/conv/depthwise_conv2d_test.py

topi/tests/python/test_topi_depthwise_conv2d.py

topi/tests/python/test_topi_dilate.py