More features & fixes.

- find_element_by_tag_name() now works.

- find_elements_by_tag_name() called with empty string gets a list of all objects; a workaround for the lack of page_source(). Also, this way you can figure out what kind of tag names to look for by doing something like

         for e in driver.find_elements_by_tag_name():
            print e.tag_name, e.text

- Added AndroidNativeKeys.py

- get_screenshot_as_base64() now works, provided write_external_storage permissions is in the AndroidManifest.xml and /sdcard/ is a valid & writable location for the png to be written to, then read back and returned as a base64 string.