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).
Done.

> In state/api/params/params.go:
>
> > @@ -387,7 +387,7 @@ type ModifyUsers struct {
> >  	Changes []ModifyUser
> >  }
> >
> > -// ModifyUser stores the parameters used for a UserManager.Add|Remove call
> > +// ModifyUser stores the parameters used for a UserManager.Add|Remove|Info call
>
> Info is wrong here now.
>
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> <https://github.com/juju/juju/pull/22/files#r13732745>.
>