Major Update: exactly identical to queue_classic

- remove the possibility to use an other table
  (I believe the table itself and the way it is declared is internal)
- topbound can be specified at Queue instantiation
- lock() returns always a result fetched using a RealDictCursor cursor
- if environment variable DEBUG is set, enable some logging
  (maybe using the lib logging would be better)
- lock() returns always all the columns
- enqueue takes only method and args in method arguments
- enqueue turns args to JSON before inserting the row
- the SQL files are exactly the same than the original queue_classic
- tests are fixed
Update: remove the hack to instantiate ConnAdapter