[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- New Basis functions having to do with dates and times, wrapped nicely 
in new
   Datetime module of standard library
- New .urp directives: 'html5', 'neverInline', 'noMangleSql'
- New command-line arguments: '-explainEmbed', '-stop'
- Changes to C FFI interface, especially for uw_register_transactional()
- 'Basis.getEnv' now always calls UNIX getenv() outside a page handler.
- Changed <active> to avoid generating an empty <span> for empty content.
- New HTML tag: <pre>
- Bug fixes and improvements to type inference, optimizations, and 
documentation