Add MurmurHash3 (unused) and misc cleanup

Added the ability to use the MurmurHash3 algorithm (but you'd have to
change some comments around to actually enable it).

Removed some duplicated lines in src/bloomfilter.h (were already
imported from src/mmapbitarray.h).

Fixed up schizophrenic type returned by _filesize(int fd).  It was
trying to return an int (-1 indicating error) and getting stored in a
uint64_t which obviously cannot even represent a -1.  So now the value
0xffffffffffffffff is used to indicate failure.

Rebuilt pybloomfilter.c with Cython 0.17.
Remove a dead-code line.