Add Python 3 support

[benmwebb]

This should add full Python 3 support by making a number of simple modifications so that the code builds and runs with both Python 2 and Python 3:

• Use print function rather than print statement;
• Use Python 3 C API protected with preprocessor macros;
• Replace except x,y with except x as y;
• Use range in Python 3 rather than xrange.

[aozalevsky]

would be very nice to see that in the master!

[benmwebb]

We incorporated this into our own fork long ago: https://github.com/salilab/pyRMSD

[aozalevsky]

@benmwebb I saw that and I am very grateful to you since I am already using it in several projects which have to be updated from the python2 to 3. it was a kind of +1 message. just in case.