Implementation of "invite a new user" modal

Closes #3079
- new controller and template for invite-new-user-modal
- actually triggers email invite via POST /ghost/api/v0.1/users/
- setting default language value (on the client) when creating a user
- only available role is "Author"
- updates to UsersIndexController to allow dynamic property calculation and template rending
While it looks very strange to have a ```<fieldset>``` element with no ```<form>``` as a parent, turns out this is actually valid: http://stackoverflow.com/questions/9812898/is-it-wrong-to-use-the-fieldset-tag-without-form-tag

The purpose of this is so that when hitting the enter-key in the input box, it won't submit the form.