resolves issue 360

See https://github.com/craftyjs/Crafty/issues/360#issuecomment-13108664

Added unit test to confirm all behavior.

Edited documentation of this.attach to be more explicit, and
particularly to mention that destroying the parent will destroy
the child.
improve destruction process for entities with attached children

In cases when this._children[i] cannot be destroyed, might as well
still delete the _parent link. (Might avoids memory leaks in some
weird cases.) Add code comment saying why the "if" is there.
Merge pull request #421 from sbyrnes321/origin/destroychildren

bug fix -- destroying parent does not destroy all children