Run whipper without installation

This commit makes it possible to run whipper directly with
`python -m whipper` from wither within the source directory
or with `PYTHONPATH` set to the source directory.

whipper/__main__.py

@@ -0,0 +1,10 @@
+# -*- Mode: Python -*-
+# vi:si:et:sw=4:sts=4:ts=4
+
+import sys
+
+from whipper.command.main import main
+
+
+if __name__ == '__main__':
+    sys.exit(main())