Adding an optional context arg to memoize.

Do you think something like this would be a good idea?

Adding a context option to memoize so that methods of an object can preserve ``this``. For example:

```javascript
var obj = {
  bar: 'value',
  foo: functions (arg, callback) {
    callback(this.bar + '!');
  }
};

obj.foo = async.memoize(obj.foo, null, obj);
```