optimize validation.js performance

it a try to optimise the Joomla! form rendering with a lot inputs.
`validation.js` it third slow script after `chosen.js` and `bootstrap.tooltip` on the form page,

Here is some graphs.
test made with disabled `chosen.js` and `bootstrap.tooltip` on Global configuration page.

Before this fix:
![screen 2014-10-02 16 23 30 1164x438](https://cloud.githubusercontent.com/assets/1568198/4493256/ea208142-4a47-11e4-9839-44ff868bd9cf.png)
After fix:
![screen 2014-10-02 16 19 55 1085x445](https://cloud.githubusercontent.com/assets/1568198/4493273/0a368328-4a48-11e4-9c66-82b366dc9109.png)

As can see difference around 400ms on my PC in Chrome.

**How to test.**
Apply patch. 
Go to global configuration, or other form where a lot inputs, and compare whether all still works and whether it "feels faster" :wink: