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. Now returns an empty results.

> In state/apiserver/usermanager/usermanager.go:
>
> > @@ -114,6 +120,51 @@ func (api *UserManagerAPI) RemoveUser(args params.Entities) (params.ErrorResults
> >  	return result, nil
> >  }
> >
> > +// UserInfo returns information on a user.
> > +func (api *UserManagerAPI) UserInfo(args params.Entities) (params.UserInfoResults, error) {
> > +	results := params.UserInfoResults{
> > +		Results: make([]params.UserInfoResult, len(args.Entities)),
> > +	}
> > +
> > +	canRead, err := api.getCanRead()
> > +	if err != nil {
> > +		results.Results[0].Error = common.ServerError(err)
>
> do we really want to duplicate the error here? once for error and once 
> in the result structure?
>
> —
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> <https://github.com/juju/juju/pull/22/files#r13732881>.
>