Added support for horizontal mouse wheel scroll

Adding support for horizontal mouse scroll for better UX.

- Tested in IE9 (should work for 10 & 11 too), Firefox v25 and Google Chrome.
- Avoided use of external libraries like jQuery to keep the code as simple as & vanilla as possible.
- Avoided different code for different browsers as much as possible.
- Avoided use of Events like 'DOMMouseScroll' for Firefox since they're deprecated (as per MDN). Used the newer 'wheel' event instead.