Uniter Action(id string) API client/server

This commit adds the ability for Uniter clients to query Actions from
state.State by id.  The response includes a server error message if the
Action is not present on the state server.
If you use backtick strings, you don't need to escape the double quote:

```go
c.Assert(actionsQueryResult.Error.Message, gc.Equals, `action "wordpress/0_a_0" not found`)
```