Used 2to3 tool for python3 compatibility. Fixes #13

[lonetwin]

Fix #13. Used the python 2to3 tool for python3 compatibility. I'm happy to report, I didn't have to do anything extra. pydown work right out-of-the-box with that.

I'm happy to contribute fixes and/or updates to this project, including refactoring, replacement of vendor'd markdown and pygments packages, if there's interest.