Reference solution directory was renamed and move to the
always-excluded-from-delivery .meta folder.

Pangrams is currently the third exercise in the new ordering therefore
it should have a starter implementation.

* make object-orientated
* add starter implementation
* rename class to PangramChecker

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.