@GordonEndersby: Now that you mention it, that does sound reasonable. I also read somewhere that when you include one of the included libraries inside of a personal library, it only includes the .h file and not the .cpp file. I'm not sure if this is entirely true, but it would explain the errors.
EDIT: I would still like to know how to succesfully do what I was trying to do, just for the sake of learning.