Reorganize dirs and switch to autotools

Reorganize the directories as discussed with @hellais. I've
not moved libneubot.py and libneubot.json, tho, because it's
not clear where to move them.

Since the directories reorganization breaks the old Makefile,
this is a good moment to switch to autotools as well.

While there, remove a couple of things (gen_cxx, neubot.hh,
libneubot_python/) that were already deprecated, to avoid
waisting brain cycles to decide where to put them.

After the above changes: the library compiles and the tests
work as before. The python-based tests do not work, however,
unless one installs the library: I've opened an issue for that.

All this work completes step one of the transition from libneubot
to libight. The next step (of which I will take care) consists of
renaming all the symbols to use the Ight (or ight) prefix instead
of the Neubot (or neubot) prefix; issue for that opened as well.