title: How to delete an Arrow in Joint JS Library?
body: <p>I've the following challenge, when a user selects an Arrow and press delete 
  key, I must clear the arrow. The arrow will be always related to 2 
  State's; when the user fires the event, I wanna know which States 
  (start and end) the arrow is related to, and wanna clear it from the 
  view. How can I do that? </p>