Let projectiles collide with objects without being armed.

With this pull request, projectiles will be able to collide with objects before they have been armed, but wont explode before they have been armed.
Old behaviour is achievable by adding an %this::explode in the onCollision callback, or simply setting a proper arming delay.
Change discussed [here](http://www.garagegames.com/community/forums/viewthread/134064/1#comment_form)