protocol v2: Version in headers

[robhaswell]

@progrium has proposed, via email, including the version number of the API in the HTTP headers. This would mean that the adapter is capable of determining the protocol without already having to know the protocol.

The downside is that this is a breaking change, but we would only need to break it once (hah).

👍 from me.

[binocarlos]

👍 Although this is a breaking change - it feels more naturally like a header i.e. meta-data equivalent to the Content-type header