Copy package file to published directory.

Addresses the comment I made here: https://github.com/geddski/grunt-release/issues/6#issuecomment-26016636

In summary:

If you use the "folder" option, then you're publishing a different directory than the one that contains your package.json file. That means that the published folder has either a stale version number or no package.json file at all. This patch simply copies the updated file over prior to publishing.