This updates our various file opening calls to explicitly open any

binary files as binary. While not a real problem on Python 2.x, Python

3.x requires this in order to get byte strings out instead of Unicode

strings.
It also ensures that any time we write a file using a Django storage

backend, we're providing a basename and not a file path, in order to

avoid issues on Django 1.11.

Unit tests pass on Python 2.7 with Django 1.6 and 3.5-3.7 with

Django 1.11 (with other in-progress changes).