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.
Fix blob responseType detection in IE.
Merge branch 'master' into haxe3

* master:
  Fix blob responseType detection in IE.
  AssetType specifies all the different formats, rename to AssetFormat.

Conflicts:
	src/flambe/platform/html/HtmlAssetPackLoader.hx