[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- Add Json module to standard library
- Make HTML5 the default and add 'xhtml' .urp directive
- Remove 'Value' attributes for AJAX-y UI widgets, because they should 
use 'Source' instead
- Change compiler to support reproducible builds, via replacement of 
timestamp
   calculation with different methods or use of content hashes
- IPv6 support in HTTP-server binaries, via '-A' command-line option
- New Top function: mapUX_rev
- Bug fixes and documentation improvements