Support creating UUIDs from more types

Cequel has a very handy helper for creating `Cql::TimeUuid` objects from at `Time` object.  However, we have a need to create UUIDs from `String` (coming from HTTP query parameters) and to a lesser extent, `Fixnum`.  Since Cequel doesn't have a helper for creating those types, we have a mix of `Cequel.uuid` and `Cql:::TimeUuid.new` calls.  It'd be nice to consolidate to one.  My preference would be within Cequel so if the UUID type changes again, fewer client code changes would be necessary.