Moving deque implementation to its own file

[bharatagarwal]

This moves the deque implementation to its own file. Also fixed a typo involving a missing backtick in the table demonstration deque operations.

Note: Removed (object) from class declaration along the lines of #112

[robot-dreams]

Thanks for cleaning this up!

[robot-dreams]

How would you feel about something like this instead?

cases = (
    (Deque.is_empty, [], [], True),
    (Deque.add_rear, [4], [4], None),
    (Deque.add_rear, ['dog'], ['dog', 4], None),
    (Deque.add_front, ['cat'], ['dog', 4, 'cat'], None),
    (Deque.add_front, [True], ['dog', 4, 'cat', True], None),
    (Deque.size, [], ['dog', 4, 'cat', True], 4),
    (Deque.is_empty, [], ['dog', 4, 'cat', True], False),
    (Deque.add_rear, [8.4], [8.4, 'dog', 4, 'cat', True], None),
    (Deque.remove_rear, [], ['dog', 4, 'cat', True], 8.4),
    (Deque.remove_front, [], ['dog', 4, 'cat'], True)
)


class TestCorrectness(unittest.TestCase):

    def test_operates_correctly(self):
        deque = Deque()
        for method, args, expected_state, expected_return in cases:
            actual_return = method(deque, *args)
            self.assertEqual(actual_return, expected_return)
            self.assertEqual(deque._items, expected_state)