Install dotenv and figure out relevant environment variables

[orenyk]

We should use dotenv to manage sensitive information like API keys and such, so let's set up this gem and compile a list of relevant variables.

[orenyk]

This doesn't really seem necessary for us (can't think of anything sensitive we're using locally at the moment), so I'm closing the issue.

[orenyk]

This is being added in #2 to allow for the use of the CAS_AUTH environment variable to switch to CAS authentication. I'm re-opening and adding to 4.0.1 (but this will be closed when #2 is merged in).