Java: grains: implement exercise

[lemoncurry]

The exercise grains has not been implemented yet for the Java track.
The description of the exercise can be found in the problem specification repository.

How to implement a new exercise for the Java track is described in detail in CONTRIBUTING.md. Please have a look there first before starting working on the exercise.
Also please make sure it is clear that you are currently working on this issue, either by asking to be assigned to it, or by opening an empty PR.

When opening an PR, please reference this issue using any of the closing keywords.

If you have had a look at the exercise description and you concluded that the exercise might not be possible to implement in the Java language, please leave a comment and describe the problem.

In case you have any further questions, feel free to ask here.

[Kineolyan]

Hello @lemoncurry, I'd like to work on this one. Cheers

[Kineolyan]

Hello @lemoncurry I forgot about the reference implementation, which I now added to the PR.
I am waiting to see if the tests are passing.

[Kineolyan]

The PR is now ready and passing all integration tests on TravisCI. I let you review it.