Allow top-level export declaration.

This prevents a SyntaxError when using export within a Module, instead
of a Script.

If the consumer needs to detect the wrong use of ExportDeclaration in a
Script, this can be done via a post-processing of the syntax tree.

https://code.google.com/p/esprima/issues/detail?id=410
Disallow nested module declaration.

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