path: added __rfloordiv__, __ifloordiv__, __iadd__.

* Removed padding """ from some lines.
* Added and fixed some comments.
path: refactored remove/copy/move methods.

* Construct Path objects with iterables, e.g.: Path(['a', 'b'])
* Consolidated similar methods into one method.
* Added more descriptive examples. Fixed comments.
* Moved walk() down a few lines.
* Added short aliases mk and cp.
path: added Paths class for lists of paths.

* Return a Paths object from some Path methods.
path: folded __floordiv__ code into __div__ funcs.

* Having one operator that handles lists and strings is simpler.
path: make use of the Paths class.