Remove git submodules
Detab update script

Death to tabs.
Modify update script to always do git checkouts
Remove force-head script

This is now done by ./update --master
README.rst: add a warning about making local changes
Adding waf support for mpv.
mpv-clean: explicitly remove files produced by old build system

This is a temporary step and currently breaks everything.

The next commit will add regular git checkouts to the update script.

Conflicts:
	ffmpeg
	mpv
README: mention what happened to force-head
README: list of dependencies is not at the end of the file anymore
mpv-clean: use distclean
clean: remove build_libs at the end

Looks better IMO.
Add ./rebuild command and suggest its use over ./update and ./build