NSMenuItem checkmarks

Adds the ability to put this in your hash:

```ruby
checked: true
```

to produce something like this:
![screenshot 2014-07-16 14 35 49](https://cloud.githubusercontent.com/assets/139261/3603561/0ad7dbdc-0d18-11e4-967d-f2a5b41f400f.png)

setting it to false and updating the menu clears the checkmark.

:sparkles: Fully tested! :sparkles: