Follow only one plane

PyMAVLink now drops any messages that do not come from the source system
specified. MAVProxy already passes its source system (from the command
line) to PyMAVLink.
Better support for target sys and comp in PyMAVLink

Added parameters for target system and component to all the mavfile
classes in mavutil. The python mavlink generator also will now generate
classes that store the target system and component and pay attention to
them. This hopefully does it right, which the last attempt (in the
previous commit) failed to do in a bad way.

Note that MAVProxy still does not provide the target to PyMAVLink, so
this does not yet do anything.
Added proper support for target sys/comp to MAVProxy

MAVProxy now passes its stored target system and component (passed on
the command line) to PyMAVLink through the proper constructors. Has not
been tested yet, so hopefully it works.
Fixed syntax errors
Added a TODO comment on barfing dropped packets
Target sys 0 = follow heartbeat source (old behavior)
Merge branch 'master' of https://github.com/uasaoc-larss/MAVProxy