Swift: add a document root container

Certain files need to reside in the root container, namely robots.txt,
crossdomain.xml and, less importantly, favicon.ico & index.html.

Add a "root" container and a rewrite.py rule that rewrites ^/[^/]*$ to
it.

Change-Id: I38b641e0f047f4d9efad88713acde6027a3e4e17