Booleanoperations

Ignore paths with zero area to prevent pyclipper raising an error (https://github.com/typemytype/booleanOperations/issues/41)

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Updated `install_requires` in setup.py to automatically resolve all dependencies upon installation, now that also fonttools and ufoLib are available on PyPI.

Added custom exceptions in `booleanOperations.exceptions` module.
These are raised if the `pyclipper` library returns an error while adding invalid 'subject' or 'clipping' contours to the clipper object, or while executing the clipping operation.
All the custom exceptions are subclasses of `BooleanOperationsError`, which for convenience is also exported in the top-level `booleanOperations` module.

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