Implement `make diagrams` command

The script scans `.c` files for state machines and draws them using
graphviz. Diagrams are not perfect, but helpful anyway. Also a quick and
dirty html page is generated.

Signed-off-by: Paul Colomiets <paul@colomiets.name>
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
make diagrams works even if destination subdirectory already exists

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Merge branch 'master' of github.com:250bpm/nanomsg