Remove event callback object when empty
Remove event callback object when empty

When there are no callbacks left attached to a system or entity, that array should be removed from the global handlers object.

Failing to do so can cause an unnecessary performance hit when a large number of extant entities have previously bound to that event.

Fixes #957.

craftyjs/Crafty,maciekwie/Crafty,maciekwie/Crafty,starwed/Crafty,craftyjs/Crafty,maciekwie/Crafty,mucaho/Crafty,starwed/Crafty,mucaho/Crafty,craftyjs/Crafty,mucaho/Crafty,starwed/Crafty