book-store: cleanup, estimate difficulty

change: estimate difficulty, update order

- Swap Bookstore for BookStore;
- Pass basket of books to method rather than constructor (simpler mental
  model);
- Remove redundant Arraylist wrappers.

perfect-numbers: set estimated difficulty to 3

collatz-conjecture: add to track

Format build.gradle files

diamond: add version comment

Rename SETUP.md to TRACK_HINTS.md

diamond: fix var name in example

Remove license info from README

This is no longer desireable, since GitHub shows license information on the repo homepage
based on the LICENSE file, provided that it can detect the license. We're using the MIT
license directly from choosealicense.com, which is maintained by GitHub, so the license is
properly detected.

sublist: remove unnecessary generic constraint

clock: tag test files with current version

clock: prefer assert(Not)Equals to assert(True|False)

Per https://github.com/exercism/docs/blob/81399dc1c87e384be9fada77f599df38f795691d/contributing-to-language-tracks/exercise-readmes.md#exercise-readmes

simple-linked-list: simplify example implementation [Fix #568]

https://github.com/exercism/xjava/issues/555#issuecomment-304126241

change: remove "final" from test class declaration

Closes: #571

* binary-search: make test class less DRY
* binary-search: use collections emptylist and singletonlist

Use idiomatic string interpolation in build file

* difference-of-squares: remove "final" from test class

* Update DifferenceOfSquaresCalculatorTest.java

acronym: update to latest canonical data

linked-list: exercise generics via tests

triangle: update starter implementation

luhn: update starter implementation

sum-of-multiples: update starter implementation

secret-handshake: update starter implementation

largest-series-product: remove "final" from class declaration

diamond: remove "final" from test class declaration

see #571