fix: add Windows support with entry_points and os.replace

- Replace os.rename() with os.replace() for atomic file operations
  on Windows (os.rename fails if destination exists on Windows)
- Add entry_points console_scripts for proper .exe generation on Windows
- Create github_backup/cli.py with main() entry point
- Add github_backup/__main__.py for python -m github_backup support
- Keep bin/github-backup as thin wrapper for backwards compatibility

Closes #112

setup.py

@@ -33,7 +33,11 @@ setup(
     author="Jose Diaz-Gonzalez",
     author_email="github-backup@josediazgonzalez.com",
     packages=["github_backup"],
-    scripts=["bin/github-backup"],
+    entry_points={
+        "console_scripts": [
+            "github-backup=github_backup.cli:main",
+        ],
+    },
     url="http://github.com/josegonzalez/python-github-backup",
     license="MIT",
     classifiers=[