#7 Implement some new variants

[Richard Archer]

Crazyhouse

I hear this is the most popular game on FICS. I imagine
much of the code required for this could be extracted
from the bughouse code so this may not be an impossible
task.

Progressive

Rumoured to be the most popular variant in OTB play, it
actually appears to be quite easy to implement.

Kriegspiel

Requested by Joey of MEWIS. This looks like an ideal
variant to play on an ICS, as the server can be the
"third person". Also sounds like a lot of fun!