Added additional logging to s3 provider

This is a reasonably trivial commit, but I thought it might help to have a bit of logging output for S3 deploys. We've also had some issues on our (really) long S3 deploys hitting the 10 minute timeout, and though the extra logs might help here?
That totally makes sense. I don't think the `verbose = false` on `log` option would work, since (I think) it would break all the existing logging from this, and other providers.

That being said, what do you think about adding the following, it involves a small change to `log`, but allows for verbosity levels, which might be kind of nice given the earlier discussion in this thread. I also figured if you're open to adding extra params to `log`, that it might be worth passing in an options hash instead.

```ruby
# lib/dpl/provider.rb

def log(message, opts = {})
  opts = {:newline => "\n"}.merge(opts)
  $stderr.print message, opts[:newline]
end

def log_verbose(message)
  case options[:logging]
  when "simple"
    log ".", :newline => nil
  when "verbose"
    log message
  end
end
```

Again, if this is all sorts of stylistically wrong - happy to go back to the drawing board here :smile: