Python 3 conversion

This converts the project to date to Python 3, with a `.python-version` pointing to `3.4.0`.

Not many notable changes -- really just updating writing of binary and text files to use different write modes. I used `2to3`, and ended up reverting its original propsals to prepend a `.` in front of my relative imports, which didn't work. (Though `from . import admin` appears to work fine.)

I also fixed a bug with some DHS reports lacking IDs, and the validations to make sure they're checking for empty strings and not just `None` values.

Last thing to check:

- [ ] Does emailing the admin still work?