Add new exercise acronym

- closes #89

- fixed a couple typos too.

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.

Fix broken image on installation instructions

Polish example linked-list exercise

Add nth-prime exercise

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.

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)