Added marker at coordinates in the url

Addresses issue #782
Changed location marker icon and animation
Fix partial traceback printing on Windows
Merge commit 'refs/pull/847/head' of github.com:overviewer/Minecraft-Overviewer into merge_847
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
Added possibility to return tuple from filterfunc

If the returned type is a tuple, it will be interpreted as first the
title (i.e. hovertext), then the content of the infowindow. If the
returned type is a string, it will be used as both the title and
infowindow content (the old behaviour). This means that older configs
are still compatible and need no changes.
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
Merge pull request #859