Add backoff and randomness to retry middleware
Make sure the post request doesn't get lost when retrying. We should update the retry to only retry posts by default though.
divorce the testing of sleep_amount from Time.now
give better failure messages for between checks
add documentation for the 2 new retry options
change interval randomness to be expressed as an float percentage
Merge pull request #296 from timgaleckas/master

Add backoff and randomness to retry middleware