Adds: Fallback if opts.attach.selector is empty

If jbox.options.attach.selector is "" ( which is the case when used like this $('p').first().jBox(...) ) it will fallback to whatever this.options.attach is set to (in that case mentioned to "$('p').first()").
Merge pull request #5 from alpadev/patch-1

Adds: Fallback if opts.attach.selector is empty