Add linked-list exercise

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.

- was: "problems", now: "tracks"
- help has been deprecated; update links.

(keeps AssertJ upgrade; reverts gradle config refactor)

Fixes #79, #80, #81.

This reverts commit 3fdc2d1b9c8749bee26140c359d93fc3a2681889.

Individual exercises must include a stand-alone build.grade, today.

Unify dependency management

Add new exercise hexadecimal

We can inject plugin, repositories and dependencies directly from
the top-level build.gradle.

This extracts a common version of junit for all projects (and
updates anagram to use assertj, the junit matchers did not work
with 4.12).

Fix example and test to use proper casing for method names

Clean up hello-world tests

Fixes https://github.com/winterchord/xjava/commit/d09fb713d19abbcbdaaa979adb59aa7fc94532fd#commitcomment-14962172

The original PR (https://github.com/exercism/xjava/pull/69) that this issue
could have been fixed in was merged before I could fix this issue.

Add hello-world exercise

Add new exercise pangram

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

Add ABOUT.md documentation to describe the Java language.

crypto-square: Fix invalid tests

allergies: Add test case to check when not allergic to anything

atbash: Add test case and sample implementation for decode.

(contributes to #53)

Needed to run the x-api, locally.

the github styles already have ruling lines for h1 and h2s.

- 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.

- without this output, another contributor might make the reasonable
  (but incorrect) assumption that the source being compiled to exercise
  the tests are from 'src/main/java'.

- 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.

Fix regex to only allow uppercased letters for the the first 2 characters.

[robot-name]

Corrected error in RandonKeyCipherTest.java