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.
The idea was that the context contains the state of the Action for return to the stateserver after completion of the action.  The Actions spec doc defines a `--failed` parameter with which the user can tell the Action to set a failure result with optional message.

The `ActionFailed` and `ActionResults` methods are there in order for their values to be retrieved by uniter when the Action is completed, in order to send them to the stateserver.

I'll add appropriate comments to these methods.