Feature/hydrators service

Provides a "hydrators" RPC service for fetching the current list of hydrator plugins available. The UI then uses this service in order to populate a dropdown for selecting the hydrator for the entity corresponding with the service. This addresses #33.

This patch also includes a new `updateHalConfig` method that more closely mirrors the style of `updateContentNegotiationConfig` et. al, and thus supercedes #35 (and fixes #13 ).