Queue

Implemented the code for the Queue data structure. Queue is a First In, First Out data structure. An empty queue is initialized with `q = Queue()`. It has the following methods `enqueue()` (adds an element to the queue), `dequeue()` (removes and returns the value at the head of the queue) and `size()` (which returns the number elements in the queue).
It would be more thorough to check that your constructor is assigning things as it should (self.tail == None, self.head == None, etc).