Add user search filter

This will add search filter to the users list.
![users-search](https://cloud.githubusercontent.com/assets/3051676/4873135/814a4b20-6203-11e4-966b-7e03ae028768.png)

Ref #489

The only problem is that the searched value is getting added to the URL query like this: `users?search-user={value}`. Not sure why and how to fix it :confused: