Allow https to be used with S3 without a cert error

The signer was always using http[s]://[bucket].s3.amazonaws.com/[object]. This doesn't work with SSL. So I flipped it to default to http[s]://s3.amazonaws.com/[bucket]/[path] so that we can use amazon's SSL cert without error.

This works for all of my uses, but might break for other users. Maybe I should write some tests to make sure all uses are covered?

Thanks for your great work!