Middleware auth bind global

this might not have been what you were thinking of doing for auth middleware but here goes:

- this adds the possibility for setting a global auth middleware (for all endpoints) or setting the auth middleware at the the individual endpoint.
- this can be used in combination with either `requireAuth` (https://github.com/dweinstein/restify-endpoints-auth/blob/dev/lib/auth.js#L29) or `endpointAuth` (https://github.com/dweinstein/restify-endpoints-auth/blob/dev/lib/auth.js#L31) which basically allows either forcing all EPs to use auth, or leaving it to the endpoint to set `auth: true`.

I'll be merging that `dev` branch in my `restify-endpoints-auth` my `master` shortly.