Add version to API

[MorrisJobke]

GET /version

{
   'version': '0.1.0'
}

cc @toto @phibos @MichaelKreil

[MorrisJobke]

Or even better something like: https://developer.github.com/v3/#current-version

[MorrisJobke]

The accept-header isn't fully compatible with the static generated JSON dump, which needs different base paths for different API versions, but should be fine too.

[yetzt]

the version header could also be sent by a reverse proxy