Singularize properties

This pull request singularizes the following properties, thereby addressing #25:

- [x] `hydra:mappings`
- [x] `hydra:members`
- [x] `hydra:operations`
- [ ] `hydra:statusCodes`
- [x] `hydra:supportedClasses`
- [x] `hydra:supportedOperations`
- [x] `hydra:supportedProperties`

`hydra:statusCodes` is not yet singularized because of #27, which needs a decision first.