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 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