Typeahead roles

Adds roles to the typeahead dropdown.

Looks like this:

![screen shot 2014-10-01 at 9 59 20 am](https://cloud.githubusercontent.com/assets/1903518/4476139/2bb04f8c-4973-11e4-944b-da5a988d916c.png)

- shows collaborators on top, but shows duplicates. We are planning on fixing that issue with the migration to select2

Example:

![screen shot 2014-10-01 at 10 00 20 am](https://cloud.githubusercontent.com/assets/1903518/4476153/4e1a06ee-4973-11e4-8005-19f02ee8e799.png)



I would agree with @mikem above.  This seems a bit dangerous since a logged in user can now very easily pull down all information from all users with very little effort.  In addition to @mikem's suggestion, it might make sense to create either a `search` method on the `UsersController` or even better, a `UsersSearchController` with a show method that does what you want.