Allow HTTP clients to determine whether a unit was created or updated

[virtuald]

This is the essentially the same bug reported as #907. #851 is vaguely related.

A HTTP API client needs to know whether the /units/NAME call created a new unit or updated a previous unit. @bcwaldon suggested that the API returns a different status code depending on which case happened. As of fleet 0.8.0, it appears to always return a 204.

Once this change is implemented, it would be great to add it to the API document too. :)

[bcwaldon]

@virtuald Are you going to tackle this, or shall I?

[virtuald]

If you guys can do it, that would be great. My golang isn't very strong, and it'll probably take you 5 minutes to do it, as opposed to my hour. :)