Allow returning details of snapshots for servers/database servers

You can set an option in either resource's snapshot request that
returns the snapshot's ID as an additional parameter.

This is extended so passing `true` to the model's `#snapshot` methods
returns (via a second API call) the image/database snapshot directly.
It's because I'm not sure if any client requires it I've been cautious and fully intend to switch to this being the default response for a stable release.

I considered just returning the identifier but wanted to offer an option for it since we have to branch the code in the request around `wrapped_request` - I'd rather not have two branches doing different HTTP work but didn't restructure all the internals for this PR. Considered it but it would have taken longer.

The other problem is there is an inconsistency already.

`Server#snapshot` returns the `Excon::Request` and `DatabaseServer#snapshot` returns `true`

I'm pretty sure I've seen parsing of the requests in one place or another but can't recall where or when. So it'll need to be a `1.0` release just for my own sanity.