issue with pickle in GitHubSession

```requests``` ```Session``` has [custom serialization](https://github.com/kennethreitz/requests/blob/master/requests/sessions.py#L664-L676), so ```GitHubSession``` must add it's attributes to ```__attrs__``` for proper serialization.

```python
In [1]: from github3 import GitHub

In [2]: gh = GitHub()

In [3]: gh.session.base_url
Out[3]: 'https://api.github.com'

In [4]: import cPickle as pickle

In [5]: dumped = pickle.dumps(gh)

In [6]: loaded = pickle.loads(dumped)

In [7]: loaded.session.base_url
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-7-c94de0900f9d> in <module>()
----> 1 loaded.session.base_url

AttributeError: 'GitHubSession' object has no attribute 'base_url'
```