Provide list of presently available buses #51

Originally reported by: Kyle Altendorf (Bitbucket: altendky, GitHub: altendky)

I just created a socketcan/PCAN only bus checker. It tries to create a bus for each interface/channel and checks for exceptions to filter out the unavailable ones.

https://github.com/altendky/st/blob/7c87276d1bb7b58e8d9c060b40d047fed5a5032d/epyq/busselector.py#L19

It's pretty hacky and hardcoded but I thought I would share it in case anyone wanted to expand on it and integrate it into python-can.

• Bitbucket: https://bitbucket.org/hardbyte/python-can/issue/51