Refactored numToWordTamil.js

Hi,

I've done a minor refactoring to the numToWordTamil.js. The core "number to words" functionality has been moved to its own method called NumAsWords. This decouples the conversion functionality from the dom requirements. This way, the user of this library can name his text values as he wishes (or not have text areas at all). 

This is a minor modification. Just to get things started.