JSON PATCH support

api-platform/api-platform#147 suggested adding a PATCH support for partials updates, it was closed as PUT already does that.

How about adding a [JSON PATCH](http://jsonpatch.com/) support? Difference here is you're able to do operations WITH entities, not only ON entites, quoted from [Why PATCH is Good for Your HTTP API](https://www.mnot.net/blog/2012/09/05/patch)

> PATCH is atomic, which means that if you need to do some complex changes to the state of a resource, you can do it with confidence. It also means that you can create synthetic resources and PATCH them if you need to orchestrate changes to the state of several resources.