Patch for Rate Limit `retry_after` Bug

Merged in pull request #49 from @dlspano with a fix to the package's rate-limit handling support where we (me) had accidentally removed the SparkApiError.retry_after attribute that is critical to handling the rate-limit messages. 🤦‍♂️ -Thank you for catching this Dave!

This release also includes a few minor commits that were added to the package in support of the ciscosparksdk work that is underway.