setuptools packaging

[bertsky]

This adds proper setuptools packaging under the (free) name pix2pixhd, using the meta-data available.

(If you want, I can make a release on PyPI, too.)

Builds on the following PRs:

• #293
• #271
• #155

Instead of having to run the scripts files via Python, this defines entry points:

• pix2pixhd-train
• pix2pixhd-test
• pix2pixhd-encode-features
• pix2pixhd-precompute-feature-maps

(I already updated the README, accordingly.)

I was able to run the test example successfully (on GPU and on CPU).