Implementing support for switching context as described here: http://code.google.com/p/selenium/source/browse/spec-draft.md?repo=mobile#133
- this feature will be used by mobile WebDriver users to switch between different contexts like the native or the webview UI element tree
- The feature is end to end tested with latest selendroid snapshot version (6a126ab3782deb7dd0cc99c6e3785c72d636959b)