OCCI Core rewrite

- changed attribute handling
  - attribute properties consist of a collection (hash) containing attribute property objects
  - attribute property objects contain all properties describing the attribute
  - attributes are derived from attribute properties and handle attribute values as input and output
  - attribute objects are derived from attribute property objects and additionally contain the attribute value
- updated all Occi classes to use new attribute classes
- introduced a new helper class for rendering Occi objects to JSON for inspect