adding search feature to users index page

Adding the search feature to the index page. User will now be able to go into the admin users page and do a query that will try to find any users that match. or partially match, on the name or email field.

### Screenshots:

#### Index page

![index](https://cloud.githubusercontent.com/assets/647255/4859845/9620aec8-60ef-11e4-9969-6f4d3708dcc3.png)


#### Search by name

![search_by_name](https://cloud.githubusercontent.com/assets/647255/4859876/e30ac57a-60ef-11e4-85c9-66c46e4fee2d.png)

#### Search by email

![search_by_email](https://cloud.githubusercontent.com/assets/647255/4859879/fae8b0da-60ef-11e4-9e12-b0a6126abf48.png)

#### Search with no results

![no_users_found](https://cloud.githubusercontent.com/assets/647255/4859889/22447a10-60f0-11e4-92c6-14ce9480712b.png)

/cc @zendesk/samson @pswadi-zendesk 

### References
 - Jira link: [Samson-94](https://zendesk.atlassian.net/browse/SAMSON-94)

### Risks
 - None
Can you do `assigns(:users).must_equal [users(:super_admin)]`?