Created Hydrators service

- Allows listing available hydrator plugins.
- Refactored Validators service in order to remove duplication between
  it and the new hydrators service.
Ensure HAL properties are updated with REST service

- Allows updating identifier_name, route_name, and hydrator in zf-hal
  metadata map for entities and collections.
- Fixes #13
CS fixes

- trailing whitespace, visibility
Pull list of hydrators from service

- and create drop-down select based on them
Ensure hydrator name is lowercased

- so that the selector can match
CS fixes

- trailing whitespace