Murmur3: fix support for bytes > 127

We only supported bytes with MSB=0.

    groovy:000> -10 as byte
    ===> -10
    groovy:000> -10 as byte & 0xff
    ===> 246
Merge pull request #85 from airbnb/pierre/plog/fix_encoding_issue

Murmur3: fix support for bytes > 127