Add a lint indicator for Interactive Linter linting

Removes default Brackets lint status indicator.
Replaces the `errorIndicator` with this `lintIndicator`.

See #61 for more detail on the reasons for this. 
The 2000 milliseconds are a magic number - but I'm not too sure how much we can do about that. `AnimationUtils.animateUsingClass` removes the animating class after this timeout or once the `webkitTransitionEnd` event fires. This leads to the animation stopping prematurely if the delay is set too low.

I think that proposing a change to AnimationUtils (or implementing our own version) if we do this often will be required in the future. For now, though, I think it is fine to have one magic number (taken from the CSS animation length). Perhaps we could use jQuery to get the length of the animation on the element.