[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- New syntactic sugar: 'let E where DS end' for 'let DS in E end'
- Add 'onChange' attributes to more tags.
- New standard library function: String.trim
- Start treating Ur/Web tag <button> as real HTML tag <button>,
   with special handling of 'value' attribute as tag content.
- Bug fixes