[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- Reimplement constructor class resolution to be more general and 
Prolog-like
- SQL table constraints
- URLs
- Client-side error handling callbacks
- CSS
- Signing cookie values cryptographically to thwart cross site request 
forgery
- Blobs and HTTP file upload
- SQL outer joins
- SQL views
- Subforms
- C and JavaScript FFI
- Path rewriting