Add d11wtq/boris as a REPL for easily testing out
some things.

- Add Boris to dev dependencies.

- Rename phpunit.php to bootstrap.php as we'll use it to bootstrap not only the tests but the environment for Boris.

- Add a Boris REPL launcher script which will load the required libraries and prepare the test DB just so they become available under the REPL.
Add methods to retrieve scoped column names.
Implement the SetValidator class.
Add a Node::isValid method.
Add tree structure assertions on tests which change it.
Fix duplicate columns check to work well within scopes.
Add more tree structure validity assertions.
Add more structure validation tests for some specific cases and operations.
Fix 'toHierarchy' sample in README.
Merge branch 'tree-validation' into develop
Extract raw/wrap/transaction method into its own helper for easy usage.
Fix static methods to not take into account scoped column values. Which will inevitably contain null values and thus restricting our queries badly.
Add isScoped istance method.
Fix root grouping by scope column values.
Scaffold SetBuilder class which will contain the Nested Set building/rebuilding functionality.
Add Node::rebuild static method.
Add two rebuild test cases.
Implement SetBuilder rebuild method.
Fix Node::getLevel computation.
Wrap rebuild operation inside a transaction.