Added keyspace.withTables, which is a promise for an object containing all the
tables/columnfamilies/whatevers. This is a convenient thing to put at the start of
your promise chains to ensure that you have the keyspace description loaded before
you call anything further.

Also added the keyspace.createTableAs(name, property, options) convenience.
This creates the table if necessary and returns the table ready to be used.
If the table already exists, it just returns it.