Add support for mobile specific ADS

Right now, the plugin only supports one AD format. However, it would be nice if we can create an AD unit then choose which AD to show on computers/tablets and an alternative AD for mobile devices.

So that if someone visits the website from a computer they see a different AD than the person on a mobile device on the same AD spot.

For example, I can choose to show 300X500 Sticky ADS for visitors on mobile phones and 728X90 Sticky ADS for visitors on computers. These two can be under one AD Unit, where I set the AD to show on computers/tablets and an alternative AD to show on mobile phones.