Action results

*Action Results* in the State
==

An Action can now be `Fail()`'ed or `Complete()`'ed, and the relevant output will be stored in the `actionresults` collection in the juju db in mongo.

The reason, or the output will be written to the `ActionResult`, along with the details of the `Action`.

The prefix of the `ActionResult.Id()` will also encode the `Action.Id()` from which it was generated (which itself encodes the `Unit.globalKey()` that the `Action` was queued for.
Do you envisage that this will return more than one Op in the future? If not then why not just return txn.Op instead of []txn.Op.
The caller could actually be cleaner if this was done (no append(), just construct the []txn.Op literally).