Fixes #1055 

* make classes and methods package private #1055

* changed nested class to public

* made Element<T> private and its constructor package-private

Reference solution directory was renamed and move to the
always-excluded-from-delivery .meta folder.

With exercism/exercism.io@54e1df3, we can now gather exercises into a
subdirectory. By separating the language track files from the
configuration and management files, it is easier to read the project.

- move gradle config under "exercises" too as it would make a mess
  otherwise.

Apply generics to the internals of Element to remove
needless casting.

Tighten Element scoping and move into Deque for succintness (sp).

Add the execise into the grade test suite.

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.