Port MurmurHash3 from C++ to C

[hajimes]

In the course of implementing hashlib-compliant interfaces (#39), I plan to port the main code of MurmurHash3 from C++ (as originally written by Austin Appleby) to C for portability.

This actually can be done with few hassles, thanks to PEP 7 updates for Python >= 3.6; versions before 3.6 had to conform to C89 and did not officially support <stdint.h> or <inttypes.h>.

In addition, I will relicense these code from the public domain to MIT. The intent is purely for resolving issues related to the public domain and its kin licenses (#43). The text of the original public domain notice will be left for attribution and recognition.

[hajimes]

The implementation of this feature has been done in version 4.0.0.