Add a file_path argument to UrlRequest.

Write the result of a request to the given file_path:

```python
from kivy.network.urlrequest import UrlRequest

UrlRequest('http://kivy.org/logos/kivy-logo-black-64.png', 'kivy_logo.png')
```

Saves the kivy logo as `kivy_logo.png` in the current directory.