module that allows one to retry a code block

 Runs a code block, and retries it when an exception occurs. It's great when
 working with flakey webservices (for example).
 .
 It's configured using four optional parameters :tries, :on, :sleep, :matching,
 :ensure, :exception_cb and runs the passed block. Should an exception occur,
 it'll retry for (n-1) times.
 .
 Should the number of retries be reached without success, the last exception
 will be raised.