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