Make Cql::Uuid behave more like SimpleUUID::UUID

One stumbling block when making the switch from `cassandra-cql` to `cql-rb` is 
the different UUID type returned from queries. This pull just adds a couple of 
method aliases to `Cql::Uuid` that makes it work with common use cases that 
were written around `SimpleUUID`:

* `to_guid` is an alias of `to_s`
* `to_i` is an alias of `value`