[Ur] smlnj build, # comments for .ur files

[Marc Weber]

These two topic topic patches are useful to me:
http://mawercer.de/~marc/urweb-mlton-smlnj-shared-main.patch

1)
This topic branch allows building the urweb compiler using smlnj.
the smlnj build of urweb is much faster than mlton.

- The common "main" code is put into main.{sml,sig}
- A --help command is added which will provide complete command line help in the
  future (incomplete)
- The {http,fastcgi,cgi}.{sig,sml} implementation was moved into settings.sml and main.sml (default protocol)
  because smlnj did not run the code at all - so no protocols were defined thus linking failed.
  This also removes some code duplication
  (code = fun .. implementation was the same in all 3 protocols except a newline)

2)
this patch implements # comments for urweb

Marc Weber