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.