Added support for extended CAN frames.

Hi.

I have written some code to add support for exended CAN frames. The selection of which frame type to use is done when the CAN object is created e.g.
bus = pyb.CAN(1,pyb.CAN.LOOPBACK,pyb.CAN.EXTENDED_FRAME)
I have also added checks on the input parameters to the send method.
Please review and comment.
Thanks
/Henrik
I think it would be better to have the argument called `extframe`, and the value a boolean.  Then you would do: `pyb.CAN(1, CAN.NORMAL, extframe=True)` for extended frames (or you can omit the extframe and just do `pyb.CAN(1, CAN.NORMAL, True)`).  This saves ROM as you no longer need to have the constants EXTENDED_FRAME and BASIC_FRAME, and saves on typing for the user :)