Hello Miller,

I'm on a project where I need to support simultaneous 11-bit and J1939 messaging. I've hacked this package a bit to make it work (it doesn't really take much) and thought I could make a contribution to provide that support.

The changes include:

1. building J1939 filters on any specified can_filters,
2. dispatching incoming non-PDU messages to a method that still ignores them but can be sub-classed for addition to the queue,
3. dispatching outgoing non-PDU messages directly to self.can_bus.send

Another change I made was to put the construction of RawCanBus into a method so a sub-class can swap in a dummy bus for tests.

Dave