Add all core exercises from Java track

[ilya-khadykin]

Java track maintainers have established core exercises for the language some of which have been already implemented in Groovy.

I propose to add the rest:

• two-fer
• scrabble-score
• secret-handshake
• matrix
• triangle
• rotational-cipher
• saddle-points
• flatten-array
• linked-list
• binary-search
• bank-account

It would be better to add separate PR for each exercise. And don't forget about a list of exercise topics and its difficulty score

[kytrinyx]

In case it is useful, the documentation for how to implement an exercise from a problem specification is here: https://github.com/exercism/docs/blob/master/you-can-help/implement-an-exercise-from-specification.md

[Dispader]

Checking off scrabble-score, just implemented by @mattmoss .

[amscotti]

Closed with PR #141, which was the last core exercises from Java track needed.