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.
I'm not sure if you want to run this all the time.  ATM we tend to run UpdateURL all the time as fireJob is rarely true.  An alternate to ensure both are run every call to Update() is to do
```
fireJob |= UpdateURL();
fireJob |= UpdateLimit();
if (fireJob)
  FireJob();
```
This is assuming that when changing the limit you want the job fired (I guess so).