Adds clear text functionality to an input box

Closes #283

* Adds jQuery plugin that adds a 'clear' icon to an input box to clear text 

---
![image](https://cloud.githubusercontent.com/assets/302258/3605976/014f9f0a-0d31-11e4-8385-a79186348274.png)
 
Should this be 
```
expect(page).to_not have_css('.search-form a.clear-input-text', visible: true)
```