Only abort XHRs that actually started loading.
Revamp XHR loader.

Retries on stalled connections should now be more robust. Downloads are
also retried on a network error.
Add missing DummySound overrides.

Fixes DummySound.dispose().
Blacklist HTML5 audio on Windows Phone.

Fixes #196.
Work around annoying AOT compiler bug.

This is just a band-aid, the real fix needs to come from the Haxe
compiler. Refs #203.
Fail the blob check if XMLHttpRequest.responseType is missing.

Looks like some older browsers don't have this field. Refs #194.
Remove unused code paths for iOS 6.

iOS 6 fails the blob check, so these became unused, whoops. We could go
back and add a special case for iOS 6, but it's gone this long without
much trouble and iOS 6 is so 2013.
Emit progress events.
Just in case, require the Blob constructor for blob loading.

Refs #194.