[Bug #758241] When you use asyncore with a non-default map, methods

[Bug #758241] When you use asyncore with a non-default map, methods
  of the dispatcher object break.  e.g. if you close() the object, it
  tries to remove itself from the default map, not from the map the
  dispatcher was created with.

  The patch, from Stephane Ninin, records the map as an attribute of
  the dispatcher instance.

2.3 bugfix candidate.