added priorityQueue functionality

added methods to JesqueService to make use of priorityEnqueue
functionality of jesqueClient
Fixed Issue#33

switched usage of exceptionHandlerRef to use the appropiate getter
instead.
udpated buildConfig

updated redis and fixed Spock dependencies
added customListener configuration field

added the possibility to add a custom worker listener in Config.groovy
added functionality to specify a custom worker and listener
increased version to 0.7 and updated README
fixed readme
fixed Readme again
moved test block back where it belongs
fixed a typo
Merge pull request #1 from peh/customListener

Custom WorkerImpl and WorkerListener
Merge pull request #32 from peh/master

Added priorityQueue, custom WorkerImpl and custom WorkerListener + a fix for #33