adds fstab plugin

This adds a plugin to expose fstab entries in the same way the filesystem plugin exposes mounted filesystems. Example output here https://gist.github.com/rillip3/72fbcb86b51b46ad202f
I was doing it the same way other plugins were doing it. See https://github.com/rackerlabs/ohai-plugins/blob/master/plugins/cron.rb#L18 for an example. I'll make the change but if we're going to push back on PRs because they don't fit the standard, the ones we already have in should really conform to that same standard.