Add setup.py to be pip installable

For https://github.com/sethoscope/heatmap/issues/32.

To test locally, note the command is simply heatmap rather than heatmap.py:

pip install -e .
heatmap test/few-points -o map.png --width 100

Notes:

• An __init__.py file isn't needed
• I don't know what name you'd like, this uses "sethoscope-heatmap" just for testing
• Version number has a .dev0 suffix to differentiate dev versions. The idea is to remove this for the release, then increment and add back. But you could leave it off if you prefer.
• You can choose classifiers from https://pypi.org/pypi?%3Aaction=list_classifiers
• No need for requirements.txt, they're included in setup.py

Coverage decreased (-0.05%) to 78.85% when pulling e7475500e0a0afa0d7e685a32be06e00f2a97e4d on hugovk:setuppable into ea0587c0be1693be5322f6b9005e84c598c6a912 on sethoscope:master.

Coverage remained the same at 78.925% when pulling 5fdb00c732e5281e40d5b6824986e05a0e1cd1f5 on hugovk:setuppable into 5cccdcdbfb0485bc16bc5928b4d718e20b76cca9 on sethoscope:master.

Coverage remained the same at 78.925% when pulling 5fdb00c732e5281e40d5b6824986e05a0e1cd1f5 on hugovk:setuppable into 5cccdcdbfb0485bc16bc5928b4d718e20b76cca9 on sethoscope:master.

Renamed to suggest heatmap-cli

It's available: https://pypi.org/project/heatmap-cli/

It would also possible to auto-deploy tagged versions to PyPI with something like this.