Separated error handler into two parts so exceptions generated while handling rejected notifications can be handled later in the pipeline. Fixes #37.
Added a note about the lingering GlobalEventExecutor thread (closes #29).
Added a link to the mailing list.
Switched to strong references for registered listeners.
Throw an IllegalStateException when registering a listener for an already-shut-down PushManager.
Merge pull request #40 from relayrides/strong_listener_references

Use strong references instead of weak references for rejected notification listeners
Merge branch 'master' of github.com:relayrides/pushy into separate_retry_queue

Conflicts:
	src/main/java/com/relayrides/pushy/apns/PushManager.java