Python package setup and basic tests

[lucaswiman]

@tabatkins This PR makes the following changes:

• Adds a setup.py file and appropriate python packaging metadata. This should allow you to upload the file to pypi^*, which would make it pip installable.
• Makes some minor Python 3 compatibility fixes.
• Adds some simple tests that run through the included examples and verify they're valid XML. This doesn't test the program logic much, but is better than nothing.
• Adds support for the tox test runner to run tests against python 2.7, 3.4, 3.5 and 3.6, showing statement coverage reports after each test run:

---------- coverage: platform darwin, python 3.5.1-final-0 -----------
Name                   Stmts   Miss  Cover   Missing
----------------------------------------------------
railroad_diagrams.py     535    195    64%   25, 27, 50, 76-77, 87, 112-113, 136, 138, 177, 217-218, 239-258, 261-287, 292-295, 298-318, 321-395, 421-422, 486-512, 515-601, 622-623, 662-663, 668-669, 681-682, 687-688, 702-703, 716-717, 733-734, 747-748, 764-765, 776-777, 790-791, 799-806

This PR also does the following:

• Adds __eq__ and a descriptive __repr__ method. I've found this makes working with the library much more user-friendly, and makes tests easier to write.
• Adds a default <style> element so a separate stylesheet is not required to render the generated SVGs. Currently the SVGs are pretty broken without the default stylesheet, so I think inlining the CSS is the right way to go:

styled	unstyled

Thanks again for writing this library!

---

^* Note: If you're unfamiliar with the upload process (which is a little confusing), I'd be happy to upload it for you, and transfer ownership of the pypi package to your pypi account. Otherwise, this is a good guide.

[lucaswiman]

I've updated the python code to add a <style> element. Annoyingly, imagemagick can't even handle that, but it seems a much cleaner solution than inlining the CSS on each element. Users of this library can inline the CSS using a tool like premailer before passing them to those tools. I don't know Javascript very well, but I'd be happy to make a similar update to the Javascript code if you'd like.

The example HTML generated by the python script is here: https://jsfiddle.net/r4cs3wkd/

[tabatkins]

Thanks for this work! I'll handle moving the style over to the JS port.