Rename all (the "neubot" prefix becomes "ight")

This is the second step of the transition from libneubot to libight: all the occurences of "neubot" have been renamed "ight" (except a few cases in which such rename does not make sense).

The only exception to the straightforward s/neubot/ight/ rule is that libneubot.py become _libight.py. In the last-week hackathon, in fact, we agreed that libneubot.py (which is autogenerated) was too low level and would have benefited some wrappers to smooth its rough edges. Since it cost nothing and since I was "renaming the world" anyways, I opted for _libight.py to reflect the low-levelness of such piece of code.

I verified that this pull request does not introduce breakages or warnings by recompiling everything. I also noticed that the Python-based tests are not working (after pull/1 the Python library is not installed anymore, plus the dynamic library name changed): however, I have pending another pull request (which depends on this one) that not only fixes the Python-based-tests regression but also allows one to run such tests from the sources directory (i.e., without installing anything). I could reorder the pull requests, if you prefer, however, I'd rather avoid that, since the other pull request already uses the new, ight-prefixed names.

After this pull request is in, there should be no stop-the-world pull requests that touch the whole tree for a while, which should allow us to octopus more parallel, narrow pull requests :-).