final edits for linklist assignments

Elizabeth and I made improvements on the code: 
* adding self __str__ method instead of defining a print method.
* Calculating the size at constant time instead of O(n) time
* Added optional next parameter to Node class 
* Empty link list checks on insert and pop methods
A proper init test would be making sure your linked list is initialized with a Node at the head containing the appropriate value (or an empty head if value is an optional parameter). I'm not quite sure what exact behavior this is testing. I do appreciate you fixing the other errors, though.