Add our standard envs.py to mitol-django-common

[rhysyngsun]

We want to pull in envs.py which provided environment variable parsing for settings.py into the common app.

Addresses feedback from #5 (comment) and #14 (comment)

A few goals here:

• Make this reusable and standardized
• Allow apps within ol-django to start defining some boilerplate settings. We'll probably accomplish that with a settings.py in each app that would get imported via from mitol.mail.settings import *