Improve GitHub events

There's a header that tells us how often we can poll and we should check the ETag for 304 header so we can decrease the number of polls we do.

Also, we should check all events since the last check, not jut the latest one.

https://developer.github.com/v3/activity/events/

Read the GitHub Events API documentation linked in the issue, then locate the code that polls GitHub events. Done means honoring the polling guidance and ETag/304 responses while checking all events since the previous check rather than only the latest event.

Written by the indexing model from the issue text.