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 don't think doing a $.hide() will degrade performance -- you're only doing it with document changes. We show/hide status bar items in file change in other places so this will have a minimal impact on performance. You could do it as a `$.once` handler to just do it the first time if you wanted.