Moved configuration files to local etc

* Moved commonly edited configuration files to
  a local 'etc' directory
Added dynamic backend loading and finalized config
Added API versioning and custom errors
Added a placeholder for the internal backend API

* The internal backend API describes communication
  between rOCCI-server core and the backend model
Instantiating the backend from an attribute
Switched to the expect syntax in RSpec (mandatory)
Added basic backend model specs
First draft of the internal backend API (stubs)
Added a skeleton for OCCI-parsing middleware
Connected rOCCI to RequestParsers::OcciParser

* Added a proof-of-concept Occi::Collection dummy