[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- Improvements to 'configure'; should now fail if any uncommon but required
   package is missing
- Other fixes to configuration, build system, and C code portability
- sigfile .urp directive & -sigfile command-line option
- .urp files with no directives no longer need to begin with blank lines.
- Other bug fixes