Restart ApsnClientThreads on thread death.

The PushManager class does not restart ApnsClientThreads when they die.  There has been a couple of bugs that has caused this (https://github.com/relayrides/pushy/issues/5 https://github.com/relayrides/pushy/issues/37).  This has left some of our hosts not sending out any notifications without any notices.

This PR just adds an uncaught exception handler to start new threads when an old one dies.  I could add something a little more robust or take a different approach all together if you have something in mind.