added function to delete all entities in the world (for example to clean a stage)
changed internal bag of systems to a list of systems, because usually systems are meant to be processed in the same order they have been added and deleting a system should not replace it with the system added last
Merge branch 'dev_fsm' of git://github.com/davidB/dartemis into davidB-dev_fsm
added a test for the EntityStateMachine and made some minor modifications
added contributors
removed the `ComponentProvider.singleton` constructor because it would not work with the current implementation of free lists
updated related documentation
EntityState : can hold ComponentModifiers, allowing to modify existing components (regardless if component was previously present or not)
EntityState (big refactor): replace EntityStateMachine by EntityStateSystem