Allow sqsQueue to be specified by its URL.

The benefit: if the queue is specified by URL, no
high-privilege AWS actions are taken to check for
the queue existence, or to create the queue in case
it doesn't exist. If you've set things up with very
limited AWS permissions, the plugin will work fine
as long as you specified correct credentials and queue
URL.

When the queue is specified by URL, only the following
permissions are needed: sqs:DeleteMessage, sqs:ReceiveMessage.
Don't get all panicky, if no SQS profiles are configured.

Prevents this error from occuring:

SEVERE: Timer task com.base2services.jenkins.SqsQueueHandler@33e5d8d0 failed
java.lang.IllegalArgumentException
	at java.util.concurrent.ThreadPoolExecutor.<init>(ThreadPoolExecutor.java:1310)
	at java.util.concurrent.ThreadPoolExecutor.<init>(ThreadPoolExecutor.java:1198)
	at java.util.concurrent.Executors.newFixedThreadPool(Executors.java:89)
	at com.base2services.jenkins.SqsQueueHandler.doRun(SqsQueueHandler.java:39)
	at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
	at java.util.TimerThread.mainLoop(Timer.java:555)
	at java.util.TimerThread.run(Timer.java:505)