Fix parsing of `import {} from "A";` and `export {};`

Previously empty import/export specifier lists wouldn't parse -- even though this is allowed in the spec:

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-imports
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-exports

This fixes the parser to allow empty import/export specifier lists

https://code.google.com/p/esprima/issues/detail?id=615