Pull settings in more directly from the environment

Use a file to specify the environment variables both for local development and for tests. We don't have to pollute the calling environment, which should help prevent running maintenance scripts against the wrong environment. We use [envrun](https://github.com/prashtx/envrun) to load the environment variables when running mocha.

This simplifies the way we read settings from modules. Before, we got ahold of the regular settings or test settings early, and then we passed around the settings object. Now, we can directly call `require('../settings')` (or similar) and be confident that we're getting the appropriate test or development settings.

/cc @hampelm