Added support for element based swiping.

This allows you to swiping within an element.  Example usage (python):
listview = driver.find_element_by_tag_name("listView")

driver.execute_script("mobile: swipe",{"touchCount":"1","startX":"0.5",
       "startY":"0.9","endX":"0.5", "endY":"0.1", "duration": "1.5", "element": listview.id})

I also added methods and utility classes to support finding absolute positions from points.