Hook to allow subclasses of GraphAPI to customise requests handling

For example if I wanted to use [CacheControl](http://cachecontrol.readthedocs.org/en/latest/) to cache the API responses, this patch allows me to have:

```python
import cachecontrol
import facebook

class CachedGraphAPI(facebook.GraphAPI):
    def _get_session(self):
        sess = super(CachedGraphAPI, self)._get_session()
        return cachecontrol.CacheControl(sess)
```