Fix chunking issue with skein256 and skein512.

Don't process the partial buffer if we can fill it completely,
if we don't know if it's the last block or not.
We let the buffer accumulate in the end, for either processing
by finalize or further update.

fix #21
Fix skein256/skein512 issue where an empty bytestring trigger state change

If an empty bytestring is use to call update, it could trigger the
processing of the buf, that could be holding the data for finalize
(the last block). That would give bad calculations.
SHA3 doesn't need to get a block for future finalization.

Process as many blocks as possible everywhere.
bump version to 0.11.3
Merge branch 'master' of ssh://github.com/vincenthz/hs-cryptohash
bump version to 0.11.4