[Ur] New release

[Adam Chlipala]

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

CHANGELOG for this release:

- Start of official tutorial
- Compiler support for generating nice tutorial HTML from literate 
source files
- New protocol 'static' for generating static pages
- Replace GCCARGS 'configure' variable with CCARGS
- Better support for compilation with Clang (including on OS X)
- Fancier 'configure' script with versioning
- Applications consult URWEB_STACK_SIZE environment variable to 
determine thread
   stack size
- HTML tag IDs now drawn from an abstract type
- New Basis functions: diffInSeconds, mkMonad, onClick, onDblclick, 
onKeydown,
   onKeypress, onKeyup, onMousedown, onMouseup, preventDefault, 
stopPropagation,
   toSeconds
- Add <image> tag
- Add 'target' attribute for <a>
- New compiler command-line option: -dumpTypes
- New syntactic sugar for computed ORDER BY clauses
- Bug fixes and optimization improvements