Add muscle powered as a fuel type.
Add a foot crank as a type of muscle powered power source.
Add a bicycle as a new vehicle type based on muscle power.
Update itypedef.cpp

Make foot crank rarer in preparation for adding it to spawn lists.
Update mapitemsdef.cpp

Added foot crank item to appropriate spawn lists.
Change schoolbus engine to medium, large was too high-performance.
Add schoolbus to rare vehicle spawn list.
Merge branch 'bicycle' into for-darklingwolf

Conflicts:
	itype.h
	mapitemsdef.cpp
	veh_typedef.cpp
Add bicycle to vehicle spawn list.