add exponential backoff and jitter

[rayrrr]

In http://developers.marketo.com/rest-api/error-codes one of the error codes states:

502 | Bad Gateway | The remote server returned an error. Likely a timeout. The request should be retried with exponential backoff.

In fact, exponential backoff is a good error handling strategy in many cases. https://github.com/litl/backoff already provides the feature; would you be willing to introduce a dependency on that package here so we could use it to decorate the execute method?

[jepcastelein]

exponential would be nice, but not sure if necessary. The library already has retry for other error message:
https://github.com/jepcastelein/marketo-rest-python/blob/master/marketorestpython/helper/http_lib.py#L42

It would be really simple to add 502 to that.

But I agree that exponential would be nice and the backoff package seems solid.

[rayrrr]

@jepcastelein thanks! I will be submitting a PR with backoff added soon. In the meantime, since I will be doing substantial work in http_lib.py I made a separate PR that is purely PEP8 line length formatting in that file first: #44 I'd like to work against that version if possible.

[rayrrr]

On second thought, don't worry about that. I closed #44 because of #35 being open and having lots of changes on the same lines.

[rayrrr]

PR #46 is ready for review. The changes removed most of if not all the too-long-for-PEP8 lines in http_lib.py anyways.

[jepcastelein]

Thanks, I just published version 0.4.1 including backoff support. I made the max_retry_time configurable, which was not as straightforward as I thought it would be. If you have any ideas on how to improve, please let me know.

[rayrrr]

Great news, thanks. I will give the matter of configuration some thought.