Code inspection improvements

Adding support (functional and partly visual) for having multiple code linters for the same document type.
This means you can have both JSLint and JSHint (it's questionable if this makes sense) linter produce linting results.

Once a linter was registered for a file type, it will run with every save operation (unless this is disabled in the UI). There is no exposed UI way to enable/disable a linter, this means at the moment "All or nothing". You can uninstall the extension that provides the linter functionality to get rid of it's warnings. It's still TBD how a UI for this could look like.

Here is how ti looks like for a JSHint and JSLint fighting for your attention:
![screenshot 2013-11-11 09 42 27](https://f.cloud.github.com/assets/86214/1514818/a232bb24-4af8-11e3-827a-0deeb3516f8b.png)