Add a keyless flag to designate commands that don't take keys

... rather than checking interval===0 which isn't true for commands with
movable keys (e.g. zunionstore).

For keyless commands we just address a random node, which was always the
intention, but this actually makes that work. Fixes #25

For those movable-keys commands we just go by the first key given. And
if redis complains then it'll fail with a redis error. Fixes #26.

This also addresses #21 inasmuch as the command shouldn't error any more.
However, MONITOR should perhaps get special treatment rather than just
addressing a random node