Adjusting exception parent class

Instead of inheriting from Exception, which is the common ancestor of
all exceptions, I'm adjusting this to be a RuntimeError.

http://blog.nicksieger.com/articles/2006/09/06/rubys-exception-hierarchy/
Moving ingest spec to conventional location

Given that lib/rof/ingest.rb is what is being specified
Then the conventional spec location is spec/lib/rof/ingest_spec.rb
Specifying high level features of ROF