Clarified some names around shutdown states.
Made the presence of a dispatch thread the indicator for "startedness." Removed spurious null checks.
Added drain-before-shutdown behavior.
Added a unit test for drain-before-shutdown behavior.
Fixed a poorly-constructed test; fixed the bug that the corrected test revealed.
Shut down the push manager quickly when using a wacky dispatch thread.
Made the no-timeout shutdown method return nothing (because the retry queue will always be empty). Updated docs.
Merge branch 'master' of github.com:relayrides/pushy into drain_retry_queue_before_shutdown_take3

Conflicts:
	src/main/java/com/relayrides/pushy/apns/PushManager.java
Made a couple deadline-handling methods static.
Minor test cleanup.
Fixed a minor synchronization goof in the mock server.
Merge pull request #63 from relayrides/drain_retry_queue_before_shutdown_take3

Add drain-before-shutdown behavior (closes #24)