ES6: Unicode code point escape

http://code.google.com/p/esprima/issues/detail?id=521
Document the support for ES6 Unicode code point escape sequence.

http://code.google.com/p/esprima/issues/detail?id=521
Remove lookahead2

https://code.google.com/p/esprima/issues/detail?id=523
Linear comment attachment.

https://code.google.com/p/esprima/issues/detail?id=522
Mention Nashorn support.

https://code.google.com/p/esprima/issues/detail?id=524
Merge branch 'master' into gh-pages