CB-2606 iOS support and additional improvements

1. Added iOS support
2. Updated FirefoxOS code
3.  WP8 and Windows8 - use default icon, if provided
4. Improved ConfigParser to support <icon> 'cdv:platform' attribute
5. Improved ConfigParser to provide default icon

The following config.xml was used for testing https://gist.github.com/sgrebnov/9949313