Updated stack.py to provide better encapsulation.

Stack class now has internal list as a 'private' member (prefixed with _)
Updated DataStructures/tests.py with tests for upcoming queue implementation.

tests.py now contains a collection of tests to test an upcoming queue implementation, using same testing style as that for stack.py.
Added queue implementation: DataStructures/queue.py

A queue is an efficient FIFO data structure. It's encapsulated in a Queue class within queue.py, alongside documentation.