Adds Deque interface to LinkedList and implements some methods.
Beginning work on #6.
Adds implementation of Deque's descendingIterator.
Working towards implementing the double-ended queue (issue #6).
Adds implementation of Deque's peeking/getter methods.
Working towards implementing a double-ended queue #6.
Implements Deque's remove methods and fixes issue #6.
Fixes #8 - wrong exception being thrown by remove when index is equal to size.