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. IsNotFound = ErrPerm, otherwise just err
>
> In state/apiserver/usermanager/usermanager.go:
>
> > +	results := params.UserInfoResults{
> > +		Results: make([]params.UserInfoResult, len(args.Entities)),
> > +	}
> > +
> > +	for i, userArg := range args.Entities {
> > +		_, username, err := names.ParseTag(userArg.Tag, names.UserTagKind)
> > +		if err != nil {
> > +			return params.UserInfoResults{}, err
> > +		}
> > +
> > +		user, err := api.state.User(username)
> > +		var result params.UserInfoResult
> > +		if err != nil {
> > +			if errors.IsNotFound(err) {
> > +				result.Error = &params.Error{
> > +					Message: fmt.Sprintf(`No user with the username:"%s" could be found`, username),
>
> Don't leak info the client doesn't need. Plain ErrPerm please.
>
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> <https://github.com/juju/juju/pull/22/files#r13637115>.
>