Added limit attribute for the directory provider

This adds the ability to limit directory content to a certain amount of items.

```
<content target="videos" limit="10">videodb://movies/titles</content>
```

Default value is 0 which will show all available items.

PS: Would be nice to have in Gotham but +1 is also fine.
You're currently using it as an integer though, so atm you have conversion in this class and in the caller?