Add Tools/fix_headers.sh

Running this script will parse the top of all source files
that are not submodules, examples or test cases, to find
all #include's and then do basically two things:

1) Reorder and group the headers so that px4_* headers are
   included first, then local headers (headers of the project
   that aren't submodules) then C++ headers if any, then C
   headers if any, then system headers (which includes submodules)
   and finally any #includes that are inside #if*...#endif
   constructs.
2) Fix the use of "" or <> in a consistent manner.

Afterwards few fixes might be necessary for compile errors that
pop up. Most of those are already fixed in my previous commits.
However, I was not able to test a compilation for ros
(with __PX4_ROS defined) -- so some more fixes might be necessary
because of the header reordering.

The script comes with a progress counter and an error summary
(if any) at the end (so no scrolling back is necessary).
It is highly recommended to only run this script in a clean
project with no outstanding changes that need to be committed.
In fact, the script enforces this (unless you pass --force).
Reverting a run of the script is then easy with 'git checkout .'.

It is also possible to run the script on a single file
by passing that on the command line. In that case it
might make sense to pass --debug too, though that was really
meant for just me, while developing the script. This will write
debug output into the file that is passed, so you don't
want to commit that! ;)