Previously we were using string concatenation to build up the directory
listing. Now we fetch some JSON from the server and use data binding to inflate
a directory listing.

This exmaple doesn't work yet because we're missing support for
template@repeat, but hopefully we'll get that soon.

Also, added support for setRequestHeader to XMLHttpRequest.

R=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/688413005