[PASS] PrecomputePrune, add testcase (#14)

nnvm/Makefile

nnvm/python/nnvm/compiler/__init__.py

nnvm/python/nnvm/compiler/build_module.py

nnvm/python/nnvm/compiler/graph_attr.py

nnvm/python/nnvm/compiler/graph_pass.py

nnvm/python/nnvm/graph.py

nnvm/python/nnvm/top/attr_dict.py

nnvm/python/nnvm/top/tensor.py

nnvm/src/compiler/packed_func_ext.cc

nnvm/src/compiler/pass/graph_fuse.cc

nnvm/src/compiler/pass/layout_transform.cc

nnvm/src/compiler/pass/precompute_prune.cc

nnvm/tests/python/compiler/test_build.py

nnvm/tests/python/compiler/test_graph_pass.py