AssetType specifies all the different formats, rename to AssetFormat.

This was necessary since saying something like:

    // Crazy php script that generates an MP3
    manifest.add("foo.php", Audio);

wasn't precise enough to figure out if the browser should load it (it
could be an unsupported audio format). Now that should be:

    manifest.add("foo.php", MP3);

Fixes #98.