Work on API improvements

[bmeurant]

Must work on Resthub REST API and clean it if needed:

• relations management
• patching ?
• hypermedia or reference support ?
• Serialization
• Param converters
• ...

partially related to #164, #45

[bclozel]

JSON views?
https://jira.springsource.org/browse/SPR-7156

[bmeurant]

This is a good start but I don't think that JsonViews are sufficient ... I think about Param converters, auto detection of id field and relation (as done in spring-data-rest & spring-data-commons) ...