Lua debug

I'm in the process of writing a Lua debugger, this PR simply integrates it with Pioneer (at least the Unix bits, which is what is needed anyway since all is done through Unix sockets)

I doubt that much people will be able to use LDB in its current state, since the "user interface" is just a python prompt, but I figured it was worth the shot to integrate it early in master in order not to have to carry the patches around :-)