Allow to manually trigger completion via $script.done

Third-party libraries on the web have their own bootstrapping logic and attaching a callback to the 'load' event of the _script_ tag is not enough.  These include Facebook, Google Maps, etc

To tie dependencies to those libraries, you need to manually attach the callbacks.  Here's what I do for Facebook:


    window.fbAsyncInit = function() { $script.done("facebook") }
    $script("http://www.facebook.com/.../all.js")


So if the Facebook library loads other scripts before initializing itself, I'm now able to wait until they finish their bootstrap to continue mine