Read settings from environment variables when not configured elsewhere

We're using logentries in a system where we try to keep configuration out of the file system and in the environment (à la [the twelve-factor app](http://12factor.net/config)).

This PR changes the "load settings" step such that, if a setting isn't configured in `Web.config` or `App.config` (or Service Configuration for Azure applications), then it's read from the environment variables.