My `FlxPreloader` changes

These changes *seem* to make the FlxPreloader work with Chrome again...

I was looking at the way NMEPreloader works (since it works on Chrome) vs the FlxPreloader, which started getting 'stuck' once my swf was > 5MB.

It looked like, at some point, someone was worried about the super.onLoaded() function being called more than once, and decided that the way to fix it was to override onLoaded to not do anything, and then call destroy once percent >= 1 and I think this seemed to work okay when the swf was small enough, but once the swf gets to a certain size this logic breaks and it was getting stuck - because EnterFrame still gets called more than once after the swf has finished loading...

Instead, I stopped overriding onLoaded, and put a flag (_loaded) that will stop EnterFrame's logic from happening after the onLoaded function is called for the first time... 
In my project, if I revert to the current version of FlxPreloader, it gets stuck every time - and when tracing I can tell that EnterFrame is being called about 10 times after the swf has finished loading.
When I use my changes, it has never gotten stuck and only goes through the EnterFrame logic until it's loaded and then stops.