Fix ES6 export parsing

It deviated from the current [ES6 draft spec](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-exports) in a few ways:

`export default class A {};` - the class in question is a class expression
`export default function () {}` - this is not allowed
`export {};` and `export {} from "foo";` - these should be allowed

Also it was kind of weird how esprima was setting the specifiers field on `export default function foo() {}` to include `"foo"`. Now the specifiers are exclusively whatever is in the `{...}` ExportClause of the export.
the discussions around this syntax came down to an overside that will be fixed in upcoming revisions (read ES7), and the error reflects what the specs are saying, the `from` token cannot be used after default, not today :)