Prevent screen sleep

Hi guys,

Since you were so kind to [add WP8 support to the Insomnia plugin](https://github.com/EddyVerbruggen/Insomnia-PhoneGap-Plugin/) I've added the plugin to your cool app.

I hope you like the changes and feel free to change anything ofcourse.

I've tested my changes on iOS, Android and WP8, all on real devices.

Thanks,
Eddy