[EXAMPLE/PYTHON] Improve extension type, add operator lib (#162)

examples/README.md

examples/operator/README.md

examples/operator/gemm/cuda_gemm_square.py

examples/operator/rnn/lstm.py

examples/operator/rnn/matexp.py

python/tvm/__init__.py

python/tvm/_ffi/_ctypes/function.py

python/tvm/_ffi/_ctypes/ndarray.py

python/tvm/_ffi/_ctypes/types.py

python/tvm/_ffi/_cython/function.pxi

python/tvm/_ffi/_cython/ndarray.pxi

python/tvm/_ffi/ndarray.py

python/tvm/_ffi/runtime_ctypes.py

python/tvm/ndarray.py

tests/python/unittest/test_codegen_cross_llvm.py

tests/python/unittest/test_runtime_extension.py