New struct, recipe, tells a Building how to produce certain resources.  Struct Recipe_amount gives us an amount of a certain recipe; Building::build_queue is now a vector of Recipe_amounts.
A couple new buildings with recipes.
Fix issue in Interface::building_status().
Improved help section.
Building::produces_resource() and Building_datum::produces_resource() now have their parameter default to RES_NULL.  If it is RES_NULL, the function returns true if ANY resource is produced.  New functions, Building(_datum)::builds_resource (returns true if the datum has a recipe that produces that resource) and Building(_datum)::get_housing() (returns the amount of housing for a given Citizen_type, which defaults to CIT_NULL, and if CIT_NULL, returns total of all housing).
Start work on allowing to alter a building's build queue from Interface::building_status().