Merge pull request #10 from emsa23/master

Towards 2.3
[maven-release-plugin] prepare release PrioritySorter-2.3
[maven-release-plugin] prepare for next development iteration
Merge remote-tracking branch 'jenkinsci/master'
Synchronized the QueueItemCache

Synchronized the QueueItemCache to avoid
ConcurrentModification and other side effects.
Made ItemInfo comparable
Added method to get a sorted list of all items.

Added method to get a sorted list of all items,
this also includes Blocked Items.

The method will return a sorted set of all known 
and active Items.
Remove item when started not when leaving Queue

Since the WeakHashMap will drop the item almost
immediately the Item cannot be removed until used
in the Throttler.