Implement ES7 Async/Await
Revert "Merge branch 'async-await' of https://github.com/sebmck/acorn"

This reverts commit 904d46ae736214d50da3a5c2b616204f0e0ae7d1, reversing
changes made to 49e421660af161af0e75c2fa066ea356d6650e69.
Merge branch 'async-await' of https://github.com/sebmck/acorn

Conflicts:
	acorn.js