Running no transfers in S3 while Grunt is in --debug mode

#22

I've made the following changes for when Grunt is running in debug mode (ie. `--debug`):

 * A warning message is shown (indicating no transfer will occur)
 * The intended action is printed, with the promise being resolved immediately

I'm not sure this is the ideal way to trigger "debug" mode, as adding `--verbose` switches debug mode off, which accidentally burned me by uploading to S3. Perhaps we should just use a `debug: true` option in the config?