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 

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![image](https://cloud.githubusercontent.com/assets/302258/3605976/014f9f0a-0d31-11e4-8385-a79186348274.png)
 
When the search text is cleared, `.search-form a.clear-input-text` element is not removed - its `visibility` is set to `hidden`