Added basic comparators to Occi::Core classes

### Includes
* `#==` and `#eql?` implementations for all key classes of `Occi::Core`
* minor fixes in `ActionInstance` rendering to JSON (`#as_json`)

### ToDo
* [x] Add comparators (`Occi::Helpers`)
* [x] Fix specs broken by those new comparators
* [ ] Add specs testing new comparators