Added option to use Kryo's CompatibleFieldSerializer

This patch introduces the new boolean parameter
msm.kryo.compatible. If this property is set to true Kryo will
use CompatibleFieldSerializer as the default serializer instead
of the ReferenceFieldSerializer which is configured in
KryoReflectionFactorySupport.

This can be used to create some compatibility between slightly
modified classes in memcached, e. g. when having minor updates to
your software.
Make creation of Kryo instance available to subclasses

In order to make the configuration of Kryo itself more
customizable the new method makeKryo is protected and can be
overriden by client code.

As the whole class is not really designed for subclassing I'm not
sure if this actually is felxible enough.