Optionally close connections after a set number of send attempts

This is (I think) the last in a series of changes intended to work around an APNs design issue where we can't know if connections are still alive. The idea here is to (optionally) close connections after a user-configurable number of notifications have been sent.

While #116 closes connections that sit idle, connections may never become idle in applications that generate lots of push notifications. This change allows very active connections to shut down after a set number of send attempts, so even applications that never allow for idle connections will have a reliable way to cycle connections.

EDIT: Changes are probably best viewed with [whitespace-only changes hidden](https://github.com/relayrides/pushy/pull/118/files?w=1).
> If the a send attempt limit is set and is less than the sent notification buffer size

Should there be some kind of enforcing that the send attempt limit, if sent, is less than the sent notification buffer size?