Retryable uns 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.