Start the rplidar motor before scanning

Also add user functions to start and stop the rplidar motor
Merge pull request #2 from funsim/fix_stopping_motor

Start the rplidar motor before scanning (by setting DTR)
Move start_motor and stop_motor into the RPLidar class; added RPLidar.motorRunning flag.
stop motor after connection
update stuct definitions in protocol.py and cmd.py to incorporate response_size, response_mode; also impose const check_bit on scan data packets