ES6 module syntax support

Provisional support for es6 module import/export syntax.

Because we use this syntax is a growing majority of our projects, I had to implement it. Obviously this doesn't track a spidermonkey implementation that doesn't yet exist, but for those that want provisional support, it could prove very useful (per the discussion in #139)

I'd love any feedback.
I unified `var-stmt`, `declaration`, `default` and `expr` into just `declaration` and `default` which correspond to the `export Declaration` and `export default HoistableDeclaration` forms respectively with 8d33e2e25865df891a4233280ad2c09bde4dfc4e.