Change create user role widget

Define defaut roles for the platform (User)
Change addUser form roles to defaut platform roles
Change addUser form role widget to checkboxes instead of multiple select

![screenshot - 090414 - 10 18 57](https://cloud.githubusercontent.com/assets/479917/2653256/48879c98-bfbf-11e3-94e9-8cfe82a162bd.png)