Document UUID class changes

It looks like cassandra-cql and cql-rb use different UUID classes.  The former used SimpleUUID (or at least Cequel used it on cassandra-cql's behalf), while the latter firmly wants a Cql::Uuid.  Having been on Cequel 0.5.6, we have SimpleUUID used all over the place.  But those UUIDs are now treated as Strings in any CQL 3.0 query.

Documenting this change would be helpful as part of the upgrade guide.  If there were a way to transparently support both, that would be even better.  I think that would necessitate change in cql-rb.  But perhaps it can be done through argument inspection in the `execute` call.