Ability to post photos and videos : multipart/form-data

Facebook require multipart/form-data as type for "source" parameter.
https://developers.facebook.com/docs/reference/api/page/#photos

```python
photo = open("picture.png", "rb")
fb_api.set("me/photos", source=photo, message="Blabla ...", published=False)
photo.close()
```

Code largely inspired from https://github.com/feinheit/django-facebook-graph