FELIX-4264 JobInfo should provide a way to identify the kind of task

* Changed Callable<T> to Job<T>, added requirement on BundleReference
* Identify 3 types of jobs
* Updated implementations and test cases

git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1528336 13f79535-47bb-0310-9956-ffa450edef68