Make accuraterip_checksum found

If `accurate_checksum` was successfully built in `src` it will
automatically be found if whipper is run via `python -m whipper`.

whipper/__main__.py

@@ -1,10 +1,14 @@
 # -*- Mode: Python -*-
 # vi:si:et:sw=4:sts=4:ts=4
 
+import os
 import sys
 
 from whipper.command.main import main
 
 
 if __name__ == '__main__':
+    # Make accuraterip_checksum be found automatically if it was built
+    local_arb = os.path.join(os.path.dirname(__file__), '..', 'src')
+    os.environ['PATH'] = ':'.join([os.getenv('PATH'), local_arb])
     sys.exit(main())