BREAKING implement (*Backend).Delete method per #1

[ahouene]

(*Backend).Delete has been implemented for all three existing backends. Corresponding tests have been added.

[wojas]

One remaining design issue is what to do if we could not delete the object because it does not exist.

The current implementation returns ErrNotExist in such case, but I think I would prefer to not return any error in such a case. All the other methods are idempotent, allowing you to repeat the same call without getting different results. Returning ErrNotExist breaks this pattern. HTTP DELETE is also idempotent.

Not returning an error in this case also makes the API easier to use. I think most code that consumes this API would end up checking for this specific error and ignoring it. I cannot think of a case where getting an ErrNotExist is useful. If you really care if it existed, you could do a List before the call.

[ahouene]

Good point and insight, I'm fixing this!

[wojas]

LGTM! We will need to release a v0.2.0 with this change.