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.

- where files are required from the "starter" (./src/main/java), these
  are now soft-linked into the example source.
- a couple examples were missing import statements.
- the example code for Hamming was failing the test suite.

At this time, these are all of the problems that have example code.

- enable compilation during CI build only for those that currently pass.
  (let's tackle the failing tests+example code one at a time).