[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- Performance optimizations for Ur/Web's standalone HTTP servers
- New command-line options for those servers: '-k' and '-q'
- New HTML pseudo-tag: <script>
- Trying to recv() from a different client's channel now triggers a run-time
   error.
- New compiler command-line argument: -print-cinclude
- Bug fixes and improvements to optimizations, error messages, and 
documentation