enables operation with 11-bit CAN messages

[DaveP987]

fixes #18

Introduces a new strict parameter to Bus that specifies whether to operate strictly on J1939 messages or J1939 and 11-bit CAN messages. strict=True by default to maintain the current behavior.

Handling of PDU vs non-PDU messages is broken out into separate methods.

So far, this seems to work fine for me as a simple drop-in replacement for can.interface.Bus using existing 11-bit messaging.

[DaveP987]

@milhead2, I'm thinking of a slight change of course to this PR. I'm coming up against some more ways that I need to be involved in message activity that would need to be implemented as a Bus sub-class. With that, I think my plan will simply be to do some simple refactoring of Bus so that my sub-class has access to hooks that it needs without altering the current behavior.

[milhead2]

Hi Dave, I'm doing some repo maintenaince for pythin-can and you have an old PR. Are you still interested in me merging it? Let me know and I'll take a harder look at it.

Miller