Fixes PooledEntity ids not being properly recycled issue #82
Adding existing entities from engine in case they where added before the system was added to the engine.
Allowing entities to be resorted on demand
renaming resort to sort
JUnit test
Adding a SortedIteratingSystem and unit tests
Merge branch 'SortedIteratingSystem' of https://github.com/Lusito/ashley into SortedIteratingSystem