Add new collection endpoints + new properties in versions endpoint

[eduardoboucas]

This PR:

• Changes the endpoints for managing collections to:
  • GET /api/collections: list collections
  • GET /api/collections/:property/:collection: view collection
  • POST /api/collections: create new collection
  • POST /api/collections/:property/:collection/fields: adds a field to a collection
  • PUT /api/collections/:property/:collection/fields/:field: updates a field's schema
  • DELETE /api/collections/:property/:collection/fields/:field: deletes a field
  • PUT /api/collections/:property/:collection/settings: updates a collection's settings
• Adds support for a fields URL parameter on the /api/collections endpoint, allowing consumers to apply a Mongo-style fields projection to the response
• Adds the fields _author and _date to the /versions endpoint, indicating the ID of the client that generated the change and the timestamp, respectively.