feat(logging): switch logging to mozlog

BREAKING CHANGE: both the config and the logging output has changed.

Closes #156
It should be unique because the op will become the `Type` in heka, and so we can filter the logs via `fxa-oauth-server.server.error.uncaught` to get only these logs.

I kept the same name because they're basically the same thing. One is an uncaught error inside a request handler, and another is an uncaught error... not in a request handler? I don't know.