Add Python 2/3 compatibility

[brian-rose]

Closes #7

Makes stuff work on both Python 2.7 and Python 3.x

Key changes:

• use the future package for interoperability. Most of the modules now have

  from __future__ import division, print_function, absolute_import

  at the beginning. This makes Python 2.7 code work the same as it does in Python 3.

• All the print statements are changed to print() functions
• all the local import somemodule statements are changed to the explicit style

  from . import somemodule

• The other difference would be the meaning of the division operator / on integers. In Python 2 this returns a truncated int, while in Python 3 it always returns a float. I didn't see any usage of integer division in the code, but you have a careful look.

Finally I included a .gitignore file in the repository to simplify the version control of code while excluding compiled objects.

[brian-rose]

@ddbkoll you should take a careful look before merging, I may easily have broken something.

I verified that the two test scripts in the repository will now both run (and seem to produce the same result) under Python 2.7 and 3.7.

But there is probably a lot of code that these test scripts do not touch.

[brian-rose]

Oh, I forgot one other change.

The string methods string.atof() and string.atoi() are replaced with simple built-in conversions float() and int().

Those methods were deprecated in Python 2 some time ago and totally absent from Python 3.