Fixing `pip install pymaxflow` build dipendences setup.

Actually `pip install PyMaxflow` is not working as expected. If you haven't installed numpy or cython, it will fail to install, instead of installing them as requirements.

Here's an example of the error:

```shell
ImportError: No module named 'Cython'
```

The reason is that setup.py, in order to work, needs to import both numpy and cython, that are not already installed, so it crashes. The only way to fix this behavior is to lazy-evaluate this two import.

I don't know if it's a very elegant solution, but it works, so...