Update YAML loading to work with Psych 4.x

Psych 4.x contains breaking changes to how YAML configs are loaded. Use the new canonical path to load config files (i.e., trusted documents), and require Psych 3.3.2+ to ensure maximum compatibility (#60).

Add TLS support

Allow TLS configuration options so that TLS can be enabled in Rabbit. (#29, @jgaxn)