[Ur] New release

[Adam Chlipala]

Available as usual at: http://www.impredicative.com/ur/

I'm still looking for people interested in building real applications, 
commercial or not, with Ur/Web.  I'm willing to devote substantial time 
(e.g., building domain-specific libraries) to specific projects that can 
help put Ur/Web through its paces.  Please contact me if you're 
interested.  I think of this as an opportunity to crush the competition 
in web development, thanks to Ur/Web's advantages in programmer 
productivity, performance, and security. :)

CHANGELOG for this release:

- Polymorphic variants (see Basis.variant)
- New 'onError' directive for .urp files
- (* *) and <!-- --> comments in XML
- Basis.classes, Basis.confirm, and Basis.tryDml
- New notations ::_ and :::_, for constructor parameters of unknown kind
- Invocations like 'urweb foo' will compile foo.ur as a single-file project,
   even if no foo.urp exists
- '-limit' command-line flag and 'limit' .urp directive
- Bug fixes and optimization improvements