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

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

* dibs : I will implement exercise bookstore

* bookstore : add to track

* bookstore : add to track

 update config.json

* Update config.json : added orc-numbers

* add missing test from the canonical spec and delete the remaning test

* using camelCase instead of snake_case for tests methods

* adding @Ignore annotation

* using List instead of ArrayList

* using instance methods instead of class method

* delete useless stuff

* modify magic numbers

* update CostPerGroup method

* update class name and files name to Bookstore from Bookstore  + using camelCase for methods

* pass the integers directly to the Arrays.asList() methods

* adding brackets

* fix constant variable

* using static for constants