Uniter API now has WatchActions

This commit adds state/api and state/apiserver endpoints for
WatchActions.
I believe WatchActions was originally on State, and the Unit WatchActions method is a minor addition, so it's possible the test was missed.  I didn't see it in here, but it appears to be implemented in state/actions_test.go: https://github.com/johnweldon/juju/commit/380d8182be2868227e80424b8e20b6f39996a66e#diff-7195d3d7d4a41d504d4c75799ca3e540R230

Should I remove this addition?