Embed sound assets, except HTML5

This pull request embeds sound assets on all targets except HTML5. Compiler conditionals ensure that the 'beep' asset is only embedded if needed, and embeds MP3 for Flash and OGG for everything else. I had to re-encode `beep.mp3` as Neko didn't like the previous sound file, and I re-encoded `flixel.mp3` as well for consistency. Essentially this just adds `FlxAssets.getBeep()` and `FlxAssets.getFlixelSound()`.