Content negotiation

[swistakm]

Now it's always JSON but we should be able to support more in a convenient way.

[swistakm]

also watch on falconry/falcon#145 issue so this may be solved within falcon framework

[swistakm]

I have removed this from any milestone because if we get basic support for this in falcon upstream. If so we will have to either fix falcon requirements on latest version or falcon. This will be backwards incompatible change anyway.