Matplotlib-venn

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- Added the possibility to provide sets (rather than subset sizes) to venn2 and venn3.
Thanks to https://github.com/aebrahim
- Functions won't bail out on sets of size 0 now (the diagrams won't look pretty, though).
Thanks to https://github.com/olgabot
- Venn2/Venn3 objects now provide information about the coordinates and radii of the circles.
- Utility functions added for drawing unweighed diagrams (venn2_unweighted, venn3_unweighted)
- Labels for zero-size sets can be switched off using a method of VennDiagram.
- Some general code refactoring.

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- Added "ax" keyword to the plotting routines to specify the axes object on which the diagram will be created.
Thanks goes to https://github.com/sinhrks

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- Fixed a bug (issue 1, "unreferenced variable 's'" in venn2 and venn2_circles)

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- Fixed a bug ("ValueError: to_rgba: Invalid rgba arg" when specifying lighter set colors)

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- Changed package name from `matplotlib.venn` to `matplotlib_venn`.
- Fixed up some places to comply with pep8 lint checks.

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- Changed parameterization of venn3 and venn3_circles (now expects 7-element vectors as arguments rather than 8-element).
- 2-set venn diagrams (functions venn2 and venn2_circles)
- Added support for non-intersecting sets ("Euler diagrams")
- Minor fixes here and there.