Reflection

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* Documentation

* core.py: All classes and methods now have docstrings
* __init__.py: A top-level docstring now exists for this file

* Improvements

* constants.py: Various error messages have been renamed and are more accurate
* core.py: Various improvements:

* Improved exception handling
* Some methods have been renamed
* Other general improvements

* Other

* utils.py: Utility functions are now placed in a separate file
* utils.py: -Associated imports are now placed in a separate file
* exceptions.py: ``TkinterException`` is now placed in a separate file

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* Bug fixes

* core.py: Include missing 'Image' in ``__all__``
* core.py: Include missing ``str.format`` calls for exceptions
* core.py: Include missing ``self`` argument in ``set_state``

* Improvements

* core.py: Alphabetically order ``__all__``
* constants.py: Replaced 'widget' with 'object' for a more accurate exception signature
* constants.py: Exceptions involving an ``Application`` instance now follow a single format

* Other

* ATTRIBUTION.txt: Added Python Imaging Library attribution text