Add  Server API method: UserInfo

Add a method to the UserManagerAPI which takes a username and returns a struct of user information (username and display name at this stage).
I converted the code to use Entities and then realised that, while it 
worked, it was hard for the reader to understand what was going on. So, 
I've made UserInfo a bulk api call with the following param structs: 
User, Users and UserInfoResult, UserInfoResults.

> In state/api/params/params.go:
>
> > @@ -741,3 +741,16 @@ type EnsureAvailability struct {
> >  	// If this is empty, then the environment's default series is used.
> >  	Series string
> >  }
> > +
> > +// UserInfo contains arguments for the UserInfo client API call.
>
> I'd expect UserInfo to just accept an Entities..?
>
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> <https://github.com/juju/juju/pull/22/files#r13437201>.
>