Supporting Tx/Rx message direction for each interface

Pull request #773 adds is_rx attribute to can.Message. Currently this attribute is set to True by default. Functionality needs to be added to each interface type to set this attribute appropriately when building CAN messages:

- [x] ics_neovi (#773)
- [ ] ixxat
- [ ] kvaser
- [ ] pcan
- [ ] seedstudio
- [ ] serial
- [x] socketcan (#780)
- [ ] systec
- [ ] usb2can
- [x] vector (#852)
- [ ] nican
- [ ] iscan
- [x] virtual (#779)
- [ ] slcan
- [ ] robotell
- [ ] canalystii
- [x] CANtact (#853)