Add PHPDoc. Cleanup and refactorization

What I've done is:
* Applied `php-cs-fixer` (for PSR-2 code style).
* Moved `TreeNode` into it's own file.
* Added PHPDoc blocks (although there is some missing documentation because I have no knowledge of the algorithm).
* Removed (what looked to be) some debugging code.
* Type hinted several functions.
* Renamed functions of `RangeInterface` (i.e. `rangeStart` to `getStart` as `range` is already implied from the name of the interface).
* Probably some small things I've forgot about.

Thank you for this interval tree implementation in PHP. :+1: