Add Stacked Configuration Files Feature

We really liked the simplicity of this gem, but for our use we need the ability to layer configuration settings.  Essentially, this allows you to provide an array of config files or hashes (the same as the original work), and uses a deep merge to combine and flatten the configurations.  I added a couple of specs and had to use a to_yaml to get the hashes to print out correctly in the error message. 

Please let me know if there are any corrections I would need to make if you were going to accept the pull request.

-M