Auto merge of #2849 - wezm:publish_dry_run, r=alexcrichton

Add --dry-run to cargo publish

This PR picks up where @JustAPerson left off in https://github.com/rust-lang/cargo/pull/1699. I've updated their changes to apply to current master and added tests, including (I think) a test that checks that the upload doesn't actually occur.

The output is as it was before:

![output](https://cloud.githubusercontent.com/assets/21787/16720362/f7ea710e-4778-11e6-8163-65f3978bb7ae.png)

Closes https://github.com/rust-lang/cargo/issues/1332