Add markers to track error lines

Keeps track of marks to prevent re-creating the same mark multiple times and so it can properly destroy marks (and their decorations) when they are no longer valid.

Should fix https://github.com/sindresorhus/atom-jshint/issues/32

It's entirely possible that I missed some use-case here, so other eyes are appreciated.
Yeah, I wasn't super-happy with that either. One way would be using `setTooltip()` on the DOM elements, which would generate something like this:

![screen shot 2014-08-05 at 4 38 25 pm](https://cloud.githubusercontent.com/assets/2036909/3818381/8d3bc6d8-1ce0-11e4-8a35-5b60a69252e9.png)

...but that's still touching the element directly, just with a different jQuery function.

Something like a top or bottom overlay might work too, but it would be a big visual departure from the previous method of showing errors.

A third option would be to just use the status bar for errors currently under the cursor (or maybe the mouse) like the jshint package for vim. Seems like we'd be losing a feature that way, though.

There is probably a way to stick html to a marker, so we could create our own tooltips, but that seems like re-inventing the wheel. 

I'll give this some more thought.