Implement queue using doubly-linked list + unit tests

[sydrawat01]

• [x] Implemented queue using doubly-linked list.
• [x] Added additional package-private methods getHead() and getTail() to get the head and tail elements in DList.
• [x] Unit tests for queue using doubly-linked list.

[xiaohuanlin]

Good job! I think your code is a compliment to the repository. But I am not if is necessary to be merged into the branch because it is just another kind of approach to implementing queue.