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.

This reverts commit 3fdc2d1b9c8749bee26140c359d93fc3a2681889.

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

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).

- 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'.

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

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.

Gradle can be run directly on Mac/Linux/Windows and
auto-download dependencies (much like sbt for Scala),
as well as simplify IDE integration.

Add appropriate .gitignore for this sub-tree.

For developers, to add a new exercism:

    cp -r _template new-exercism
    # create test in new-exercism/src/test/java/SampleTest.java
    git add new-exercism

Users will be able to just run "./gradlew check" to run tests.

To use this template to create a new assignment,
simply cp _template to the name of your new assignment
and drop in the appropriate tests.