hydra:Error is of type hydra:Class and not rdfs:Class

It is also a subclass of hydra:StatusCodeDescription.
Remove unnecessary subclassing of hydra:Operation

It is already implicitly a subclass of rdfs:Resource.
Explicitly state that classes whose instances are hydra:Resources are subclasses thereof
Fix range of hydra:operations
Singularize properties (PR #34)

The following properties have been singularized:
 - hydra:mappings
 - hydra:members
 - hydra:operations
 - hydra:supportedClasses
 - hydra:supportedOperations
 - hydra:supportedProperties

hydra:statusCodes is not yet singularized because of #27, which needs a decision first.

This closes closes #34
Singularize properties in vocab illustration and schema.org draft

This completes the singularization of properties done in 352de435b1ea1e718f9ac87960d9fc9dd9043246 and closes #25.

hydra:statusCodes is being further discussed in issue #27.
Add acknowledgements

... at least a first version thereof.
Merge branch 'master' of github.com:HydraCG/Specifications

- hydra:Error is of type hydra:Class and not rdfs:Class
- Remove unnecessary subclassing of hydra:Operation
- Explicitly state that classes whose instances are hydra:Resources are subclasses thereof
- Fix range of hydra:operations
- Singularize properties
- Singularize properties in vocab illustration and schema.org draft
- Add acknowledgements