Doesn't support async functions

Consider:

```javascript
const objectHash = require("object-hash");
objectHash(async function foo () {}); // Error: Unknown object type "asyncfunction"
```

This breaks usage with untranspiled code (node v7.4.0 and above as well as any ES2017+ compatible browser)

I've added a PR: https://github.com/puleos/object-hash/pull/68