Two small changes

Set `self.app` in `init_app` so that a simple example like this will work:
```python
from flask import Flask
from flask.ext.assets import Environment
env = Environment()
app = Flask(__name__)
env.init_app(app)
env.debug = False
```
(Will throw `RuntimeError: assets instance not bound to an application, and no application in current context` without this).

Return value from webassets `main` so that deployment scripts can catch errors.
Example: https://github.com/paulegan/flask-assets-dist-example/blob/master/setup.py#L21