Updates and corrections (now works on Android)

* updated the selection of the menu into a var inside main.js so the
menu item is not queried over and over again.

* on Android devices the menu wasn't showing since, in contrast to IOS,
Android understands the "contextmenu" event, thus firing the same stuff
twice. Added now a device-check part to the respective event to cancel
it when the page is called on a mobile device. (Snippets from
http://DavidWalsh.name)

* Updated Readme to show the new corrections

* Tested now with iPhone 3GS using native Safari + Nexus 7 using chrome