Support a user-supplied Requests Session

The instance of Stash I use is Kerberized. We have a [custom authentication mechanism](http://docs.python-requests.org/en/latest/user/advanced/#custom-authentication) for Requests that does the right thing; so this patch lets me tell stashy to use it:

```python
session = requests.Session()
session.auth = MyKerberosAuth()
stash = stashy.client.Stash(url, session=session)
```

I think this also addresses at least part of issue #4.

I've not tested this with a regular username + password-flavoured Stash, because I don't have one on hand.