allow tables without no id and no primary key

VERSION

@@ -1 +1 @@
-Version 2.4.6-stable+timestamp.2013.05.12.20.06.49
+Version 2.4.6-stable+timestamp.2013.05.13.10.11.03

gluon/dal.py

@@ -8216,7 +8216,7 @@ class Table(object):
         self._plural = args.get(
             'plural',pluralize(self._singular.lower()).capitalize())
         # horrible but for backard compatibility of appamdin:
-        if 'primarykey' in args and args['primarykey']:
+        if 'primarykey' in args and args['primarykey'] is not None:
             self._primarykey = args.get('primarykey')
 
         self._before_insert = []