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.

Start by running the examples/flask_sqlalchemy example and checking the GraphQLView.as_view usage shown there and in the README. Reproduce the GraphiQL request that raises the Request.get exception, then verify the context handling and ensure the example works out of the box or clearly documents the required configuration.

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