Simplify the interface by making the return value of CQLClient.execute a

Deferred that fires with an iterable of row dictionaries.

In CQL3 the key will never be anything other than an empty string. See:
[ResultSet.java][1].

In addition `Column.timestamp` and `Column.ttl` will
always be null, and to access the ttl or timestamp you should use the
`WRITETIME(column_name)` and `TTL(column_name)` functions.  See: [writetime][2].

[1]: https://github.com/apache/cassandra/blob/08df0b8604cd34c4f1fb384f9f6eaba86203d116/src/java/org/apache/cassandra/cql3/ResultSet.java#L135 "ResultSet.java#L135"

[2]: http://www.datastax.com/docs/1.2/cql_cli/using/writetime "Determining time-to-live and write time."