Acorn v2.6.x compatability. Implement async `for-in` loops

• Correctly asynchronize ES6 for...in loops.
• Update the plugin code to remove 'async' and 'await' from the super-strict keyword tests introduced in acorn v2.6.x that generate parse errors before the plugin gets a chance to manage them. Also compatible with acorn v2.5.2 as used by previous versions of nodent.