Kryo 4.0 branch maintainence release

See all the changes here:
0.9.2...v0.9.3

This should be binary compatible with 0.9 and 0.8 (excluding the binary incompatibilities of Kryo 4 vs Kryo 3).

This includes:

• performance optimization by @jcdavis #290
• registering of BigDecimal serializer by @andrewsmartin #312
• tests to ensure serialized data is not broken by @gdiet #310
• a QTreeSerializer by @nevillelyh #305
• VolatileByteRef serializer by @johnynek #308
  and many other

Thanks to all contributors!