add "clearAllCache" option (all cookies) to Childbrowser plugin

it is small patch concerning this issue: https://github.com/phonegap/phonegap-plugins/issues/563

#### Sample usage: In your own app's javascript file
``` 
 window.plugins.childBrowser.showWebPage("http://www.google.com", {  clearAllCache: true });
```

since it is my first time doing pulling request, please let me know if there is a specific format or if any further information is required.