[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- Cygwin compatibility
- Compatibility with Gentoo packaging process, including a .ebuild file
- Change typing of SQL subqueries, to indicate that they may always 
return NULL
   (for no rows)
- Syntactic sugar for GROUP BY with variable numbers of columns
   (using 'tab.{{c}}', where 'c :: {Type}')
- 'ALL' for SQL relational operators
- Add nullable types to the class for valid operands for SQL arithmetic
- 'alt' attribute for <img>
- <sup> and <sub> HTML tags
- Allow 'debug' and 'naughtyDebug' in client-side code, implemented with 
'alert'
- Bug fixes and optimization improvements