avi: Fix file writing

Hi Randy - you mentioned movie writing wasn't working for you.  This
commit fixes it (at least on my linux box).
Add first cut of droneapi integration

Hi Randy:
Seems to work okay with my odroid (except see all the FIXMEs for where
to plug in the image stuff).  Extensive docs at beginning of
balloon-strategy.py.

See the docs for details but the code that changes vehicle mode and sets
guided dests should be disabled unless you manually enter guided mode
(for safety during testing).  However, you'll certainly want to test on
SITL first.

I slightly refactored find_balloon.py so I could share code from the
api module.

I'm happy to help more as needed, just ping me on skype...

Sample session:
kevinh@kevin-think:~/development/drone/ardupilot-balloon-finder/scripts$ mavproxy.py --master=192.168.1.69:14550
Logging to mav.tlog
Running script /home/kevinh/.mavinit.scr
-> module load api
DroneAPI loaded
Loaded module api
MAV> GPS lock at 122 meters
ARMED
RTL> Mode RTL

RTL> api start balloon-strategy.py
RTL>
FIXME - add image analysis and fancy-pants math
Not driving to Location:lat=21.3907359,lon=-157.74843,alt=123.349998474,is_relative=None
FIXME - add image analysis and fancy-pants math
Not driving to Location:lat=21.3907359,lon=-157.74843,alt=123.349998474,is_relative=None
FIXME - add image analysis and fancy-pants math
Not driving to Location:lat=21.3907283,lon=-157.7484363,alt=123.349998474,is_relative=None
FIXME - add image analysis and fancy-pants math
Not driving to Location:lat=21.3907283,lon=-157.7484363,alt=123.370002747,is_relative=None