Make pip installable and add CLI entrypoints

[cthoyt]

This PR closes #5 by putting the code in a directory structure that's required by python packages, by adding a minimal setup.py/setup.cfg.

It also closes #6 by encapsulating the main functionality (that used to live under the if __name__ == '__main__' block) with functions, then by adding the [options.entry_points] header to setup.cfg.

I also added an example in the README on how to run the code on one of the examples presented in your paper. I'm a big fan of DeepWalk and related methods, and I appreciate the incredible work you did to push the theoretical side of it. I hope this PR is helpful for you, and many others!

[sudoLife]

How unfortunate that the authors have abandoned their work. Thank you for your efforts @cthoyt!

Btw perhaps you managed to pin down the requirements.txt for this? Because the API of NumPy has changed and so finding the exact version of everything is tedious.