Mixer for my flask app gives error when used with db.init_app() -- AttributeError: 'module' object has no attribute 'extensions'

Hi,

I am trying to use `mixer` with my flask app to create fake data. I have written below mixer module

```
# module: my-mixer.py
from flask import Flask
from mixer.backend.flask import mixer
from faker import Faker

from database import db
from models import Country
from recruitment import app

app = flask.Flask('myapp')
db.init_app(app)
db.app = app

fake = Faker()

mixer.init_app(app)

# Generate a random country
country = mixer.blend(Country)

# Generate a Country. Set country name.
country = mixer.blend(Country, country__name=fake.country())
```

The db is taken from

```
from flask.ext.sqlalchemy import SQLAlchemy
# no app object passed! Instead we use use db.init_app in the factory.
db = SQLAlchemy()
```

You can see that I am not directly binding app with db

```
db = SQLAlchemy(app)
```

Now when I run **my-mixer.py**, I get below error

```
Traceback (most recent call last):
File "faker-mixer.py", line 16, in
mixer.init_app(app)
File "/home/hussain/workspace/test/venv/local/lib/python2.7/site-packages/mixer/backend/flask.py", line 45, in init_app
assert app.extensions and app.extensions[
AttributeError: 'module' object has no attribute 'extensions'
```
##

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Reproduce the failure by running my-mixer.py with the db defined in database and initialized through db.init_app(app). Start at mixer/backend/flask.py line 45 and compare that expectation with the Flask app setup shown in the issue. Done means the example no longer raises the AttributeError and the db.init_app() configuration remains supported.

Written by the indexing model from the issue text.