modal windows on Mobile Devices

Today, not really possible to manage Joomla 3 administration on mobile device.
The reason : modal windows not usable!

![3169 modal windows on mobile devices](https://f.cloud.github.com/assets/2385058/2253125/6fa0249c-9db6-11e3-935a-bbfbcc76f3c1.jpg)

This PR resizes Modal Windows on small device, and gives user scrollability on mobile devices.

Issue Tracker [#33328] : http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33328