Retry stalled asset downloads in HTML.

Which seems to happen occassionally on mobile browsers. This
periodically checks if the last progress event is more than 5 seconds
old, and if so, aborts and retries up to a total of 4 attempts.

Closes #14.
Firefox fixes.
Ugh, iOS 6 doesn't seem to actually support the blob responseType.

So, no progress events and timeouts on image downloads there sadly. I
could base64 encode the response string and shove it in a data URL, but
that's gross.
Fix for multi-touch event detection.

Huh, this used to work...