[Ur] Making Ur/Web applications serve static files

[Adam Chlipala]

By popular request, I've added an Ur/Web feature to serve static files 
from application executables.  The feature is in the latest development 
version:
     http://hg.impredicative.com/urweb

See file tests/files.urp for example usage.

Folks who've requested this sort of feature: it would be great if you 
could let me know if it does what you wanted.  Then I can make a new 
official Ur/Web release, once everything seems to be in order. Thanks!