Introduce constraints and name mapping to source plugin database.

- Using default values now instead of NULL to signal empty field
  because NULL values are considered distinct in the context of
  UNIQUE table constraints in sqlite
- Introduced new table for mappings from sid, game, server and team
  to channel name i preparation for new functionality
- Updated and expanded unit test

modules/source/source_test.py

@@ -202,7 +202,7 @@ class Test(unittest.TestCase):
         self.assertEqual(self.mm.serverCB['callback'], self.s)
     
     def resetDB(self):
-        self.s.db.db.execute("DELETE FROM source");
+        self.s.db.db.reset()
         
     def resetState(self):
         self.resetDB()