Added custom MIME types for OCCI

* text/occi             => :occi
* application/occi+json => :occi_json
* application/occi+xml  => :occi_xml
Updated dummies and added basic conf support
Added configuration sample
Added dynamic strategy loading for Warden

* Strategies are now configurable in 'config/config.yml'
* Discovery and loading of strategies at runtime from
  'lib/authentication_strategies'
Added stubs for hooks and backends

* Configuration stubs (YAML)
* Implementation stubs (Ruby)
Re-worked configuration loading, added more stubs

* Configuration stubs for hooks and backends
* Smarter configuration loading and presentation
Automatically enabling registered authN strategies
Updated dependencies
Moved to a more reasonable backend structure

* One backend loader as a model
* Flexible backend structure, no forced separation into
  compute, storage, network etc.