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.
I thought it was important to have a test that demonstrates that the map merge code functions properly, as in charm/actions_test which has a similar map manipulation method.

I see what you mean though!  [Roger wilco](http://en.wikipedia.org/wiki/Voice_procedure#Words_in_voice_procedure)