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. Removed.
>
> In state/apiserver/usermanager/usermanager.go:
>
> > +
> > +// UserInfo returns information on a user with the username in args.Username.
> > +func (api *UserManagerAPI) UserInfo(args params.UserInfo) (result params.UserInfoResult, err error) {
> > +	username := args.Username
> > +	user, err := api.state.User(username)
> > +	if err != nil {
> > +		if errors.IsNotFound(err) {
> > +			result.Error = &params.Error{
> > +				Message: fmt.Sprintf(`No user with the username:"%s" could be found`, username),
> > +				Code:    params.CodeNotFound,
> > +			}
> > +		} else {
> > +			return result, err
> > +		}
> > +	} else {
> > +		// TODO(waigani) add ID
>
> I don't think we need to put ID in the params type if we don't yet 
> expose it (or have any clear idea how it'd be used if we did expose it)?
>
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> <https://github.com/juju/juju/pull/22/files#r13437474>.
>