[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- Allow any FFI module to declare new HTML tags
- Ignore polymorphism in JavaScript calls to custom FFI functions, 
allowing a
   kind of simple dynamic typing (unsafe, of course)
- Add some name-mangling rules to allow XML attribute 'name' and 
attributes with
   dashes
- New Basis members: pow and tryRpc
- Add 'placeholder' attribute for <password>
- New standard library functions: Option.unsafeGet
- Add ./configure MLTONARGS option for MLTON arguments
- Bug fixes and documentation improvements