Simplified username/token retrieval

Instead of:

```python
username = flask.session.get(
        app.config['CAS_USERNAME_SESSION_KEY'], None),
token = flask.session.get(
        app.config['CAS_TOKEN_SESSION_KEY'], None),
```

There is now two new properties in the cas object :

```python
username = cas.username
token = cas.token
```
Merge pull request #6 from cameronbwhite/add_get_username_method

Simplified username/token retrieval