Add memoryview support in _crc32c

[khvn26]

#72 allows users to compress memoryviews via snappy.compress interface.
However, StreamCompressor.add_chunk raises TypeError when generating crc:

env36/lib/python3.6/site-packages/snappy/snappy.py:139: in add_chunk
    crc = _masked_crc32c(chunk)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

data = <memory at 0x7fb4e4cad648>

    def _masked_crc32c(data):
        # see the framing format specification
>       crc = _crc32c(data)
E       TypeError: argument 1 must be read-only bytes-like object, not memoryview

Looks like snappy__crc32c should be refactored to also support the buffer protocol.