Use setup.py develop for installation

[rshin]

This should have the same effect as updating PYTHONPATH. I suppose all the other install-*.md would need to be updated too...

[robertnishihara]

Nice! This is cleaner for deployment, but a little more work during development, right (because it's no longer enough to recompile, you also have to rerun setup.py)? That's probably the right tradeoff though.

In addition to updating the other installation documents, could you also update the .travis.yml and dockerfiles to reflect this new workflow? And maybe remove the setup-env.sh file as well.

[pcmoritz]

Richard pointed out to me that there is python setup.py develop which does what we want for development (i.e. you don't need to re-run python setup.py if you edit the source), see http://stackoverflow.com/questions/19048732/python-setup-py-develop-vs-install

+1 for switching to this workflow