Add support for Faktory MUTATE Api

[Pavieł Michalkievič]

There are two ways to mutate server state: queue actions (pause, resume, clear) and via operations on the mutable sets (kill, discard, requeue, clear) jobs in a target set, where a target set is one of "scheduled", "dead", "retries"). We are currently supporting a subset of queue actions command and will support of them once #59 is landed.

See more details in the Faktory's wiki for Mutate-API as well as
the Go bindings reference implementation.

Also see how the sets stats will become available as part of the Faktory's current state structure once this PR is landed.