Add linked-list exercise

[counterleft]

The tests assume that the Deque class is implemented with generics.

The example implementation keeps track of a circular head Element.

Completes one task in #3.

[jtigger]

Thank you! Looks really good, @winterchord. Thanks for following suit and using AssertJ! Also, this is a great exercise to introduce generics; nice touch.

I'm not sure how strict we want to be with the progression. This should definitely be later in the track as you've placed it. I do notice that in the check list (#3), it's after Pascal's Triangle.

[counterleft]

@jtigger Yeah, I wasn't sure exactly where to place it in the progression. I tried to place it relative to where it was in the ruby progression. I didn't even think about using the checklist in #3 as the progression order.

[sit]

Element should be generic as well and instead of storing an Object should store an object of type T. Then this cast becomes unnecessary.

[counterleft]

You're absolutely right. I had that in a previous implementation. Don't know why I got rid of it.

[sit]

I don't know what the other tracks look like, but there should be tests for popping and shifting off an empty Deque.

[counterleft]

The instructions for the linked-list exercise (https://github.com/exercism/x-common/blob/master/linked-list.md) state the following:

To keep your implementation simple, the tests will not cover error conditions. Specifically: pop or shift will never be called on an empty Deque.

[counterleft]

I'll create another PR for with the suggested changes and cc you @sit.