Add nether brick slab support
Added manual POI option

Here's how to use it:
have a list called "manualpois" in your config filled with objects that contain
the attributes id, x, y and z. Feel free to add more attributes as you need them.
Then, write filter functions to filter for your custom POIs (hint: use the id).
manualpois is now a per-render option
Don't print scary traceback when textures cannot be found

See #860
Converted the IOErrors i missed before
Merge pull request #858
Better manualPOI validator