Add BaseStation and MobileStation classes

- BaseStation includes a control channel transmitter and a receiver
- Added control protocol data structure

TODO
- Add control protocol parser (sync and grab the control frame, and
  invoke BaseStation to process it.
- Add agent list for BaseStation. Each agent handles traffic on one channel.
- Mobile station should start hand shake (Request Walsh code)
- Add control protocol parser for MobileStation (obtain Walsh codes)