Modified serial_link.py to record SBP logging data.

serial_link.py can now log serialized, timestamped SBP messages to a
consistently-named logfile, but only if libsbp is available for
serialization.

r=@fnoble,@cbeighley

Closes #186.
Cleanup serializaton logger.

Moves log formatting (timestamps) into actual callback.
Bit of refactoring for SBP logging.

0. Added fallback SBP definition since libsbp isn't out yet and its
use here is completely superfluous.
1. Reverted msg_type callback interface.
Merge pull request #193 from mookerji/183-spectesting

Modified serial_link.py to record SBP logging data.