Add method to numpy::dtype to set byteorder

I've taken a look at `numpy/dtype.hpp`, and there does not appear to be any way to set the byte order of the array being constructed. The workaround I am currently using (which requires adding the numpy C API to my build and including the headers in my source) is the following:

```
np::dtype scalarDtype = np::dtype::get_builtin();
reinterpret_cast(scalarDtype.ptr())->byteorder = '>';
```

So it should be as simple as adding a method to dtype to allow the user to set the byteorder on the underlying numpy struct.