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 wrapping of `messages` in an array (`[messages]`) is weird, but since I want the messages passed into the handler in `lintIndicator` as a single array, this is the best way to do it.

jQuery "spreads" the contents of the array over the arguments of the handlers. That, in turn, results in messages being passed into the handler "spread" over the `arguments` array. The `[messages]` code prevents that from happening, as jQuery then spreads the "new" array and as a result passes `messages` as the first argument.