Examples don't work

Hi,

I followed the flask-sqlalchemy examples [here](https://github.com/graphql-python/graphene-sqlalchemy/blob/master/examples/flask_sqlalchemy) (and the snippets in the project's Readme as well) but it didn't work out of the box. I had the following exception: `AttributeError: 'Request' object has no attribute 'get'` when trying to request from Graphiql.

After some fiddling with flask-sqlalchemy i solved the bug by passing a `context` argument to `GraphQLView.as_view`.

Here is my full url declaration:
```python
app.add_url_rule(
'/graphql',
view_func=GraphQLView.as_view(
'graphql',
schema=schema,
graphiql=True,
context={'session': db_session}
),
)
```

I don't know if this is the proper way to fix the problem, but thought it'll be useful to tell you about it.