Just re-lookup; close our end of the connection, maybe.
Use a priority queue for messages.

This should prioritize log messages by log level, so more severe messages should be sent first; if the queue gets too full, messages with less severe priorities should be expunged. Log statements with identical levels should be sorted by timestamp (newest first).

* GelfMessage.java: implement comparable.
(integerLevel): new field.
(<init>): new constructor, set `integerLevel` if passed an integer (to cache the integer value of log messages; so we can compare faster).
(compareTo): new method.
* GelfMessageFactory.java (makeMessage): pass in integer level value to constructor.
* GelfTCPSender.java (messageQueue): change to a `MinMaxPriorityQueue`.
(queueLock, queueCondition): new fields.
(<init>): initialize queue with natural order, max size.
(<init>): use lock and condition when fetching from the queue.
(sendMessage): use lock and condition when queueing messages.
* pom.xml: downgrade Guava to 13.0.1, include in compile scope (I'm using this for Cassandra, that's the version of Guava I have).
README update.