[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- New .urp directive: 'jsModule'
- New compiler command-line option: '-js'
- New HTML attribute for <button>: 'disabled'
- Allow inexhaustive patterns for lefthand sides of top-level 'val' 
declarations
- New standard-library functions: 'List.appi' and 'Option.app'
- Support for Emacs bg-build mode
- Bug fixes and improvements to error messages