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.

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

Use Guava in the tests to make it easier to build up
maps needed for testing the API.