action-set: a jujuc command for Actions to send values back to state

action-set permits action hooks to define an arbitrary map of params
which will be returned to the state server at the end of the Action,
or to specify --failed with an optional failure message.

DEPENDS: #617, https://github.com/juju/charm/pull/43
Addressed in 617 PR comment: https://github.com/juju/juju/pull/617#discussion_r17270511