Update setup.py to expose only snappy and _snappy as top-level mo…

[kronosapiens]

When trying to add this package as a dependency, our build tool began giving strange errors: it thought that this package would meet all requirements.

This was because the earlier setup.py had packages = ['.'], implying that this package provided everything. This PR updates setup.py to explicitly expose only snappy as a package provided by this library.

[martindurant]

Thank you, sounds very reasonable. package_dir probably isn't even required (although I don't think it can hurt).

[kronosapiens]

I added package_dir to avoid having to move the source code into a snappy package directory (setup.py was failing initially because it was looking for an inner snappy package). package_dir tells distutils that the snappy directory is really just root.