python3 compatility, many bugs fixed, new features

1. I added python3 compatibility for download_pdbs.py and dlpisces.py. (Hopefully these are the last remaining scripts that were incompatible with python3. Warning: I have only tested download_pdbs.py. I have not yet verified that dlpisces.py still works.)
2. I fixed a bug in coords2dihedrals.py
3. I added the ability to calculate "projected dihedrals". (The angle between the infinite lines formed by the first two and last two atoms, respectively.)
4. I fixed numerous packaging bugs that were preventing installation via pip.

There may still be many remaining packaging bugs and python3 compatibility bugs I have not yet discovered. However if the scripts are able to run without crashing, then the calculation of angles and distances should be accurate now.