deprecate relation(class, table) syntax

[Mike Bayer]

people are more often tripping over this thing:

   m = mapper(class1, table1)
   m2 = mapper(class2, table2, properties = {
     'foo': relation(class1, table1)
   })

since its not easy to see that relation(class1, table1) creates another new mapper. just get rid of that style; they can specify the mapper or the class by itself and thats it (or maybe just the mapper).

[Mike Bayer]

fixed in changeset:890