Show line-specific error in status bar

This change shows the jshint error for the current line in the status bar, instead of the first error. I also added a handler to update the gutter on scroll, since it looked like that was getting redrawn whenever I scrolled more than a few lines. There's probably a better event to use, but I couldn't find it.

No worries if you're already working on this and don't want to merge. I'm having fun just poking around in Atom.