World performance

fixes #7

Removed the redundant check() calls from process.

Bugfix in addEntity() / deleteEntity().

When adding an entity and removing it right after that, systems & managers would get notified the other way around, first of the deletion and THEN of the addition.
Fixed that by an additional if-clause in deleteEntity() that checks if the entity was just added, and then prevents the addition by removing it from the added bag.

Doesn't include the removal of Bag#clear() in World#check(), I guess that will come merging in from your less-gc branch.

I'll write some unit tests tommorow.

cheers.