add documentation, scons check for jpg support in sdl_image

It was reported on Dec 22 2014 in irc that the game is broken if
this is missing, and a search shows that bigmaps and many campaign
story screens use jpg images.

It makes more sense imo to document this as an official dependency
now, and if we decide to change everything to png's then we can go
back and disable the jpg check then.

Conflicts:
	SConstruct