Update timer handling to support multiple events at the same time.

This can occur when servicing two software serial ports via a multiple
timer channels on the same channel.
Avoid softserial loopback on port 2 on Olimexino boards.
Ignore incoming software serial data that doesn't have a start and stop
bit.  Track software serial transmission and receive errors.
  
Nothing exposes the error counter values yet.
Merge pull request #59 from hydra/softserial-track-errors

Softserial track errors and be more strict with incoming data.