Add support for encoding relationships as resource identifiers

[neomerx]

When relationship resources are not to be added to included section it's not necessary to read all resource fields and type + id (resource identity) should be enough. Schema should have an ability to encode relationships this way. It will reduce number of requests to database to load required data.

Also check one more time how greedy parser with resource relationships reading

[neomerx]

Got first working iteration. It needs more testing and documentation updates.