add call to a custom destination validator

Signed-off-by: Cristian Stoica <crististm@gmail.com>
make each king aware of each-other

Signed-off-by: Cristian Stoica <crististm@gmail.com>
make kings stay away from each-other

Signed-off-by: Cristian Stoica <crististm@gmail.com>
stash
Merge branch 'develop' of https://github.com/crististm/chess into develop

Conflicts:
	board.py