Change to addComponent

A change to addComponent, as discussed [here](https://groups.google.com/forum/?fromgroups=#!topic/craftyjs/JMPNyq61KlA).

* Only toggles the __c[id] flag when the component is actually added.
* Doesn't add the component if the flag is already true.

The second change might break some code that was relying on the old behavior, but is clearly the better behavior.