ES6 import/export syntax support

Partial implementation of #1001.

This PR was targeted mainly at supporting the [ES6 module transpiler](https://github.com/square/es6-module-transpiler) project, and thus doesn't implement the `module` block statement. I'd love to do this in the future, especially once implementations of it appear in the SpiderMonkey/V8, but for now, import/export should be good enough. Import also adds labels for imports & properly supports `undef`.

Let me know what you'd think. It'd be great if we could get a merge in ASAP with further additions later on, but I'd be up for further polishing if you don't think it's in a good enough state.