Overhaul concept of mix-ins (ingredients)

 - Base potion (MateriaPrima) now takes care of adding, initializing and
   disposing of ingredients
 - Ingredients do not inherit from alchemy.core.Ingredient anymore, any
   object can be a mix-in
 - allow simple mix-ins (fast) and advanced ones (slower but conflict
   free)
 - Removed obsolete alchemy.core.Ingredient
 - Tests, tests, test...