Image-bbox-slicer

* Split `main.py` into `slicer.py` and `resizer.py`
* Changed the demos and docs accordingly
* Code meets `pycodestyle` standards
* Changed the title

* Works seamlessly on Windows now

* Fixed some bugs.

* Fixed an issue related to optional tags in xml files.

* Added image and box annotation resizing feature.
* User can now choose to ignore empty tiles i.e. tiles with no labels.

* More PyPi issues fixed