Support export default = { .. } in esnext

- Add check for "=" token after default keyword and advance it.
- Add test cases to es6 test fixture and update unit test.

Based on discussions from es6-module-transpiler and esprima
(https://github.com/square/es6-module-transpiler/issues/63?,
 https://code.google.com/p/esprima/issues/detail?id=410) it seems like
both syntaxes should be supported.