Add envClientSessionBackend, ENV-based session key

This can be useful if:

1. You can't rely on a persistent file system (e.g. Heroku)
2. Your application is open source (e.g. you can't commit the key)

By keeping a consistent value in the environment variable, your users will have
consistent sessions without relying on the file system.

Usage:

    makeSessionBackend _ = fmap Just $ envClientSessionBackend 120 "SESSION_KEY"

Related: https://github.com/yesodweb/yesod/issues/806

Totally open to bikeshedding the function name.