Email error logging; transports set with NODE_ENV

Refactor the loging configuration into a single file, ``lib/logging.js``, which can be required by any module that wants the standard logging configuration.  This eliminates the need for ``unhangout-server`` to hold a public reference to the logger and assign it to other components, and removes the many invocations of winston.

Eliminate the logging transport and level options sent to unhangout-server's ``init``.  Instead, use the NODE_ENV environment variable to determine transport and level options.  Use the following NODE_ENV conventions:

  - NODE_ENV=production : log to file at level INFO, and send emails on error.
  - NODE_ENV=testing : log at level CRIT to console.
  - else: log at level DEBUG to console.

``npm test`` now invokes ``NODE_ENV=testing mocha``; invoking mocha directly without setting the ``NODE_ENV`` will result in default console logging during tests.  In production, the NODE_ENV=production
environment var must be set to configure logging properly.

Pertinent to issue #130.