ios webview caching

Partial work for @imurchie to take over.
ah i see. so what was confusing was the use of the word 'callback' when not referring to a callback as in a JS callback function.

i can see a motivation for something like this:

```js
// callbackAddress and callbackPort are used for executeAsync to communicate with Appium;
// see parser.js for details on these server params
```

what do you think about that?