[Tilesathome] Tileset file support

[Matthias Julius]

I have just committed the last pieces of the tileset file support for
the client.  The _unstable client should now create proper tileset
files according to the specs
(http://wiki.openstreetmap.org/index.php/Tiles%40home/Tileset_as_one_file_v2).

I have also updated this wiki page to clarify a couple of things and
to add the special case of files containing only empty tiles.  Please
comment if there is anything unclear.

The client still won't process requests from the server if
CreateTilesetFile is set until the server is fully supporting the file
format and has been tested.

Matthias