Tests and example implementation based on the ruby exercise.

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

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.

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

- where files are required from the "starter" (./src/main/java), these
  are now soft-linked into the example source.
- a couple examples were missing import statements.
- the example code for Hamming was failing the test suite.

At this time, these are all of the problems that have example code.

- enable compilation during CI build only for those that currently pass.
  (let's tackle the failing tests+example code one at a time).

- adjusted "etl" as it has "starter" code.

- move example.java so we can refer to it in Gradle as a soureSet.
- Java requires public classes be contained in identically-named files.
- using latest Travis infrastructure (better, stronger, faster).
- each piece of travis config has been confirmed as minimum necessary at
  this time.
  - without JDK specified, would run with default (Java 7)
  - without TERM=dumb, travis output is unreadable.

We will be relying on people installing gradle themselves. The documentation
on help.exercism.io has been updated with instructions for both Windows and Mac.