Allow 'default' for exported variable declaration.

https://code.google.com/p/esprima/issues/detail?id=410
Allow 'default' for exported function and class.

https://code.google.com/p/esprima/issues/detail?id=410
Support parsing 'export default = <assignment expression>'.

https://code.google.com/p/esprima/issues/detail?id=410