Add handling of linter messages when no linters for a file type exist (r...

...esets the linter indicator).

Fixes #66.
I would probably the following:

``` javascript 
var linterManager = null;


linterManager = {
    registerDocument: registerDocument,
    registerLinter: registerLinter
};

return linterManager;
```

Then you can just `triggerHandler` right on `linterManager`.