This is a) cooler and b) a requirement for deploying it on PyPI.
It removes need of __all__ shit (which is hard to keep updated), and
allows installing instaloader easily as a global module and executable.
Additionally it removes __init__.py.
This commit allows doing `import instaloader` when instaloader is
located in a subdirectory "instaloader". This makes it easier to use
instaloader e.g. when it is imported using git submodules feature.
