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. 
I'm not sure if this is the right way to load these. It works on my computer, but may not work elsewhere. Also, these are the default Brackets' symbols. Do we want to use the lightbulb with a background, perhaps?

(Same goes for other status-classes).