Adding StructureOverlay an overlay to color the map according to structures

Differences to my earlier Mineraloverlay extension pull request:
  * While writing documentation it occured to me that the extended MineralOverlay was too complex
  * I split of from MineralOverlay the StructureOverlay
  * I moved the default input from C where it was quite awful to python where it is much nicer

So actually this closes the former pull request #1125.

As for the code here it is:

A usecase to demonstrate a possible application of the extended functionality:

**"Rails Overlay that draws only the rails that are on Cobblestone for a subway map."**

With this patch it is very easy to achive that:

```python
MineralOverlay(minerals=[(((0, 0, 0, 66), (0, -1, 0, 4)), (255, 0, 0, 255)),
                         (((0, 0, 0, 27), (0, -1, 0, 4)), (0, 255, 0, 255))])
```

In this case the overlay will be red for rails on cobblestone and green for powerrails on cobblestone.
The syntax is `(<tuple of conditions>, <target color>)`
 * where `<target color>` is a 4 tuple with a `(r, g, b, a)` color
 * and `<tuple of conditions>` is a tuple with an arbitrary number of conditions with the following syntax:
`((relx, rely, relz, blkid), ...)` where the `rel<>` parameters specify the relative coordinates to the block that is checked if it matches bklid.

In the example the fist tuple `(0,0,0,66)` checks if at the current position is a
rail while `(0,-1,0,4)` checks if at one below the current position is a cobblestone.
If both are true then the color `(255, 0, 0, 255)` is used.

A Sample Config file exploiting the capabilities:

``` python
worlds['My World'] = "~/.minecraft/saves/test/"
outputdir = "/tmp/test_render"
rendermode = "lighting"

renders["render1"] = {
    'world': 'My World',
    'title': 'A regular render',
}
renders["render_overlay_dafault_rails"] = {
    'world': 'My World',
    'title': 'Default Rails',
    'rendermode': [ClearBase(), StructureOverlay()],
    'overlay': ['render1'],
}
renders["render_overlay_cust_rails"] = {
    'world': 'My World',
    'title': 'Custom Rails',
    #relative coordinates [[(relx, rely, relz, mineral)], (red, green, blue, alpha)]
    'rendermode': [ClearBase(), StructureOverlay(structures=[(((0, 0, 0, 66), (0, -1, 0, 4)), (255, 0, 0, 255)),
                                                            (((0, 0, 0, 27), (0, -1, 0, 4)), (0, 255, 0, 255))])],
    'overlay': ['render1'],
}
```

The "Default Rails" overlay uses default coloring of the structures overlay. "Custom Rails" uses some custom coloring.

fixes overviewer/Minecraft-Overviewer#556 and fixes overviewer/Minecraft-Overviewer#787