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.

A straight port of the Ruby tests. The example implementation
uses Joda Time which is generally considered a saner date/time
library than what is built in to Java.