Add small webserver for development

When working on documentation it can be helpful to use a browser. In
some environments it's not possible to access the files directly from a
graphical browser. This trivial webserver exports all files in the
current directory and unlike alternatives such as thttpd, doesn't check
for world-readable permissions.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>