Convert the directory listing to run on top of the platform

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