Adding a SortedIteratingSystem

Different approach to #74
I don't extend IteratingSystem, as I use EntityListener to fill the list.
Sorting only happens when something actually changed.

Edit: Using libgdx Array/ImmutableArray
Can you please remove the comments on `private` members. I know other parts in Ashley use it, I'm planning on getting rid of those.

Comments on self documenting code are redundant.