NCBC-359 Allows Serializer customization

This patch allows serializer customization by placeing a static field
on CouchbaseClientExtensions.JsonSerializerSettings that enables users of
the couchbase extension class to customize the serializer.

Change-Id: I009ae4db9dd7fd24f813bde3727ab3e79f6b8750
Reviewed-on: http://review.couchbase.org/32280
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>