Tests and example implementation based on the ruby exercise.

Completes a task for https://github.com/exercism/xjava/issues/3.

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 https://github.com/exercism/xjava/issues/3.

Based on code review feedback.

This implementation does not generate a test suite based on the json
meta the hello-world exercise.
See https://github.com/exercism/todo/issues/13

Fixes https://github.com/exercism/todo/issues/35