* What is the expected output? What do you see instead?

I am expecting to see the Python equivalent of dragAndDropOn(), but it's not supported
for the Python WebElement type.

For more info on dragAndDropOn, see:
http://seleniumhq.org/docs/09_webdriver.html#using-drag-and-drop

* What version of the product are you using? On what operating system?

selenium-2.0b1 in Ubuntu

* Please provide any additional information below.

There is already a browser command in remote/command.py called dragElement. Does dragAndDropOn
use that? If so, then all that needs to be done is to create the API method that sends
that command to the browser.