bin entrypoint structure

[mlocati]

At the moment, bin/concrete is pretty useless: it's only role is to call bin/concrete.php

What about:

1. deleting bin/concrete
2. renaming bin/concrete.php as bin/concrete. making it executable, and setting its shebang as #!/usr/bin/env php?
3. add a Windows entrypoint bin/concrete.bat like we have in the core repo?