Methods Client#list_users and Client#invite_user added + work from @mikedougherty

Hey guys!

We have added 2 methods for [inviting users](http://support.hockeyapp.net/kb/api/api-team-members#invite-user) and [listing them](http://support.hockeyapp.net/kb/api/api-team-members#list-users). Also added some changes to README.md.

Can you please merge this code into master branch?

Thanks!

- [Oto Brglez](https://github.com/otobrglez)