Improve publish API

This pull request has a few changes:

 - Base path is now a required parameter up front, with no default value.
 - The options parameter is now optional, allowing the module to be used like this: `ghpages.publish(basePath, callback)`
 - I've removed the JSDoc comments since the `publish` function is now variadic and I'm not sure it's worth the trouble. Happy to be convinced otherwise :)