Convert python 2 scripts to python 3 scripts

[zxdawn]

This commit resolves #39.

[behr-github]

I appreciate the contribution, but I need to be able to verify that it does not break existing functionality and remains compatible with Python 2 and 3 so as to not break existing users' workflows. I will leave this open until I have time to write a test suite.

[zxdawn]

@behr-github Thank you. I suggest that you can use sys.version_info.major == 3 to check the version. If that's true, just run 2to3 -w . in the rep directory. Maybe, you need time to test whether all of these functions work fine in py3.