Fix Python 3 issues

[barentsen]

I propose a few changes to make f3 compatible with Python 3:

• change xrange into range;
• use relative imports inside the package (import files => from . import files);
• ensure indices are integer.

Finally, I propose adding kplr to install_requires in setup.py, to help users hit the ground running; setup.py will automatically install kplr using pip.

(I was also going to remove the pyfits dependency but speedy @mirca beat me to it!)

[benmontet]

I like it.

Only issue with kplr is I believe the version you need hasn't been uploaded to pypi. We should verify/prod Dan if this is still true.

[barentsen]

I removed kplr from install_requires in setup.py!