Breaking all twitter classes out into their own file.

This is done without breaking exsiting functionality or imports by doing imports
in the `twitter/__init__.py` file.

`user.py` imports `Status` and `status.py` imports `User`
In order to prevent cyclic import errors, they are imported only if they are
actually used.

Also, sys was not imported in the `setup.py` file, causing `setup.py` to fail if
setuptools can't be imported.