Final improvements before 2.0.

 Change log:
 
 - MobileElement has become abstract. It implements
FindsByAccessibilityId
 
 - AndroidElement and IOSElement have been created. 
 
 - AndroidElement implements FindsByAndroidAutomator.
 
 - IOSElement implements FindsByIosAutomation.
 
 - Both AppiumDriver and MobileElement implement ScrollsTo. Yeah!
MobileElement is scrollable too!
 
 - PageFactory facilities are synchronized with changes above.