extra(): Re-implemented. Now is polymorphic (nearly) without restrictions.

Added test cases + docs.

pexp/management/commands/pcmd.py

@@ -30,11 +30,10 @@ class Command(NoArgsCommand):
         print Project.objects.all()
         print
 
-        """
         ModelA.objects.all().delete()
         a=ModelA.objects.create(field1='A1')
         b=ModelB.objects.create(field1='B1', field2='B2')
         c=ModelC.objects.create(field1='C1', field2='C2', field3='C3')
         print ModelA.objects.all()
         print
-        """
+

pexp/models.py

@@ -13,7 +13,7 @@ class ArtProject(Project):
 class ResearchProject(Project):
     supervisor = models.CharField(max_length=30)
 
-class ModelA(ShowFieldType, PolymorphicModel):
+class ModelA(ShowFieldTypeAndContent, PolymorphicModel):
     field1 = models.CharField(max_length=10)
 class ModelB(ModelA):
     field2 = models.CharField(max_length=10)