ekzhu/datasketch: hashfunc to replace hashobj

Now support hashfunc parameter for MinHash and HyperLogLog. The old parameter hashobj is removed.

# Let's use MurmurHash3.
import mmh3

# We need to define a new hash function that outputs an integer that
# can be encoded in 32 bits.
def _hash_func(d):
    return mmh3.hash32(d)

# Use this function in MinHash constructor.
m = MinHash(hashfunc=_hash_func)