Use only one form of per-environment configuration

:fork_and_knife: 

This is a reprise of #298, rebased on the current master branch. Supermarket's configuration parameters are now specified solely in environment variables, with [dotenv](http://github.com/bkeepers/dotenv) being the default way to do so.

In most cases, replacing references to `Supermarket::Config` was a matter of substitution. In a couple of places (the SegmentIO agent and the Paperclip configuration come to mind) there was a small amount of additional work required.

I also took this opportunity to remove all but the most essential parameters in the default `.env` file, and to switch the format of `.env` to the environment variable style. All tunable parameters, whether or not they appear in the default `.env` file, should be documented in `docs/CONFIGURING.md`.

Finally, in the process of removing and documenting the StatsD configuration parameters, I decided to remove StatsD altogether until we have a specific reason to use it.