Add mobile 'source' getPageSourceXML

There's no XPath for JSON. The XML is much easier to deal with. This adds a `mobile 'source'` method which returns XML on Android and the regular source on iOS.

```ruby
> mobile 'source'
post
session/1227006c-1b72-4215-bfbe-8c79064fb053/execute
{"script":"mobile: source","args":[]}
=> "<?xml version='1.0' encoding='UTF-8' standalone='yes' ?>...
```