Added SDL backend (mode 5) and a small, basic tileset.

There are some known deficiencies:

1. Font size is not configurable (and may be small).
1. Font offsets scale with tile sizes; this may cause unacceptable gaps in font rendering
1. Tile graphics are subpar (I'm not an artist)
1. Input is pretty hackish at the moment (since SDL doesn't deal with letter case, apparently).
1. Discrepancies between key names according to ncurses and key names according to SDL have not been thoroughly tested.
1. Input is buffered (when not in realtime mode), causing potentially slow actions like AI calculations to buffer an unacceptably large amount of input commands.
1. Config file syntax is presently very fragile and verbose.
1. Most of this is undocumented.
We don't link against the sdl or ncurses in the server now