Add ajax search against /users/search?q=

TODO:

- [x] store the user id instead of the full name
- [x] increase the width of the multiselect box
- [x] load initial list of authors
- [x] load user fullnames
- [x] Support default list, adding new users, and refreshing the metadata

Screenshot:
![image](https://cloud.githubusercontent.com/assets/253202/2895080/6e5fab5c-d55b-11e3-83ea-45af656fe2c1.png)