Implement version recognition in Accept header

[saimaz]

We should require format: application/vnd.ongr.v3+json

[trandangtri]

@saimaz, in case this format is required, so how do you think about the configuration:

ongr_api:
   version_in_url: true

IMO, it's useless and we could remove it

[saimaz]

This config was left for legacy support if user can't handle versions via headers and leave them in URL. IMO would be nice to support both ways in URL and in Header.

[saimaz]

For 1.0.0 lets keep version in the header. Later we will figure out how to implement it in both ways.