Enable star images to be set to absolute path

I noticed that it is currently not possible to set the starOn, starOff, and starHalf images to absolute paths because of the assumption that it is always a relative path.

So I changed the code so that if the path option is set to null, then it does not append a trailing slash to the path but rather sets the path to an empty string.

This enables absolute paths for the star images to be passed into the options, which is important if you are using a CDN for serving up your assets.

Example:

    $('.container').raty({
      path:     null,
      starOn:   'http://cdn.myhost.com/img/star_on.png',
      starOff:  'http://cdn.myhost.com/img/star_off.png',
      starHalf: 'http://cdn.myhost.com/img/star_half.png'
    });