Fix ExportDeclaration field types and default values.

The main confusion here was that AssignmentExpression means something
different in the spec from what it means in the AST type hierarchy, where
it actually has to involve an assignment operator like = or +=. In the
spec, an AssignmentExpression is just a slight restriction of the general
Expression type, so that's what we should be using here.

Closes #28.
Merge pull request #29 from benjamn/fix-ExportDeclaration

Fix ExportDeclaration field types and default values.