Makefile.in: install libneubot.so.0.0 and its symlinks
gen_py: load major version 0 of the dynamic library
.gitignore: do not ignore libneubot.py and neubot.h

It's better to have them in tree, so that we know when we
break either the API or the ABI.
Add libneubot.py neubot.h

Therefore, we know when we break the API and/or the ABI, because
we see that these files have changed.