Implement more NATS Auth methods

Add new auth methods for connecting with NATS clusters:

• user/password auth
• token auth

Support YAML syntax for config files

Config files can use YAML syntax now

Better MongoDB configuration

MongoDB driver supports configuring a Replica Set

Add Service Discovery

Merge pull request #38 from netlify/salvador/add-dns-discovery-package

[discovery] Add DNS discovery methods. Rabbit and Nats config to support DNS discovery