Adding CAN filter management

(Second pull request. The first failed in travis due to a DNS error)
Added some code to be able to configure CAN filtering. 


I would prefer that the can.configfilter method (maybe it should be named setfilter() in this case?) implicitly enables the filter and then adding a method can.resetfilter(number) that resets and disables it. Using the words enable and disable somewhat implies that the state and configuration of the filter is stored somewhere. The HAL does not have any function to only enable or disable a filter bank, even if the hardware allows that.