Added protos.ENV_PRODUCTION

This variable is set when NODE_ENV is set to 'production'. You can
also launch the app in production mode by editing the boot.js file.

When deploying on the live server, the application is usually run
(or at least it's recommended) using:

  protos server --env production

This sets the NODE_ENV variable to 'production'. Having a distinction
between the production mode triggered by NODE_ENV and the one triggered by
the boot.js file, you can easily test on production-like environments
without having to change any configuration.

This allows fine grained control on which configuration to use
on each of the production modes.