XML: added MAV_CMD_NAV_SPLINE_WAYPOINT

This is used to create a spline path waypoint
pymavlink: raise version
pymavlink: draw spline waypoints on map
common.xml: added SERIAL_CONTROL message

this message allows for control of a serial device on a remote
node. This can be used to (for example) update the firmware on a 3DR
air radio by connecting to a Pixhawk over USB.
pymavlink: added MavlinkSerialPort class

this class gives remote serial control of a device via SERIAL_CONTROL
messages
pymavlink: raise version