Added support for YAML files.

This pull request adds support for parsing YAML files using the SnakeYAML code. It needs to be tested.  I am not 100% sure that it works I do not completely understand the typesafe/config design. 

The reason for this modification is to make it easier to introduce HOCON into environments that use YAML.  With this change, a HOCON file can include a YAML file. At least that is the intent! :)