Allow upload headers

This adds a new option on the upload block in the config, allowing headers to be passed through to pkgcloud with the same syntax they use:

```
cloudfiles: {
  prod: {
    'user': 'your Rackspace username',
    'key': 'your Rackspace API key',
    'region': 'DFW',
    'upload': [{
      'container': 'name of your Cloud Files container',
      'src': 'source/static/**/*',
      'dest': 'some/folder/',
      'stripcomponents': 1,
      headers: {
          'access-control-allow-origin': 'something',
          'content-disposition': 'something-else'
      }
    }]
  }
}

```

It is useful if you need to override the file headers, although you might ideally want more granular control over each file