Change script to load async

[TalAter]

Change the script to load asynchronously, as suggested by @slightlyoff.

[AkshatM]

What is wrong with the async attribute in HTML5 for this?

I understand Opera support is missing, but not sure what other shortcomings abound.

[TalAter]

I don't think that will be enough, as the other methods (e.g. UpUp.start()) won't work before the script loaded.