OCCI Core rewrite

- [x] Merged existing pull request gwdg/rOCCI#79
- [x] 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
- [x] updated all Occi classes to use new attribute classes
- [x] introduced a new helper class for rendering Occi objects to JSON for inspect
- [ ] Action handling must be fixed with regards to triggering actions on remote Occi entities
- [ ] adding and removing Mixins must be implemented using the new attribute class
- [ ] get_class must be fixed
  - replace all characters which are not [a-z] and [0-9] with UTF-8 character names
  - Implement changes to OCCI Core document (e.g. parent …)
- [ ] Implement interoperability fixes in Text Parser
- [ ] fix existing spec tests and add new tests for new classes