Adding refactored SBP code, moved from piksi_firmware.
SBP - unit tests for callback registration
SBP - handle case where len = 0 better in send_message.
SBP - unit tests for send_message
SBP - unit tests for sbp_process
SBP - fix bug where state was not reset if there was an error.

(yay for unit testing)
Merge pull request #6 from fnoble/sbp_refactor

SBP refactor