Allow for ignoring particular events

This change allows you to return `nil` in an event_retriever to ignore a
paricular webhook event. You might want to do this for security
reasons (See #36). You might want to do this for ignoring test webhooks
in production (See #33 and #34).
Added example docs to readme about ignoring events
Merge pull request #38 from rmm5t/ignore-event-support

Allow for ignoring particular events