document mapper primary_key better, custom column properties

[Mike Bayer]

two more big ones that come on the list a lot, being able to do this:

   m =mapper(class, table, properties= 
     'foo':table.c.foocol
   })

to override column names, particularly when they want to make a setter/getter property around it, and this:

   m = mapper(class, table, primary_key = [table.c.col2](table.c.col1,)))

to specify custom primary key columns.

[Mike Bayer]

changeset:2225