Add blob ajaxTransport

This allow us to make AJAX requests with a "blob" dataType and get raw
binary data back. This will be used in later commits to retrieve the
thumbnail images directly.

XMLHTTPRequest Level 2 support is needed for this, which means that
this only works in IE 10, Firefox 28, and Chrome 33. See

  http://caniuse.com/#search=xhr2
  http://www.w3.org/TR/XMLHttpRequest2/
  https://developer.mozilla.org/en/docs/Web/API/XMLHttpRequest

for more information.

Code taken from https://gist.github.com/mgeisler/93d02db0d972925b086d.
Send extracted thumbnails data directly to stdout

The data is no longer base64 encoded. This saves time since we don't
need to start a Python ZeroVM job for this. It also saves some
bandwidth since base64 encoding expands the input by 33%.

This change breaks compatibility with IE 9 since we now require
support for XMLHttpRequest Level 2:

  http://caniuse.com/#search=xhr2
  http://www.w3.org/TR/XMLHttpRequest2/

Some 81% of the users have this support, and for a bleeding-edge
project like ours, I believe we can require a current browser.
Extract thumbnail 30 sec into the clips

This normally gives a more interesting thumbnail.