Refine adding collaborators

:fork_and_knife: This makes two refinements to the add collaborators feature. Firstly it allows you to search by chef account usernames. Secondly it moves collaborators/new into a modal using the reveal ajax feature. The collaborators/new view felt a little sparse and lacked context. I think the form is simple enough to warrant moving it into a modal and it has more context now.

![screen shot 2014-04-23 at 8 34 20 am](https://cloud.githubusercontent.com/assets/316507/2777344/d9b1fd74-cae3-11e3-93f6-5a51dcfeb675.png)