Fix complex ES6 destructring, export async function
- Fix issue with non-trivial destructuring assignments (fixes MatAtBread/fast-async#8 - thanks to @simonbuchan)
- Fix case where empty "else" block throws an exception (fixes #50 - thanks to @Antender)
- Fix case where single line exported function was incorrectly hoisted with no reference (fixes MatAtBread/fast-async#7 - thanks to @simonbuchan, @naturalethic and @nightwolfz)
- Bump acorn to v3.3.0