Merge init() and init_app()

I don't get the difference between ```__init__()``` and ```init_app()```, because the script seems to only set a callback via ```after_request()``` in the Flask app.

Besides, if you do the following, the script crashes:
```Python
from flask import Flask
from flask.ext.compress import Compress

compress = Compress()

def start_app():
    app = Flask(__name__)
    compress.init_app(app)
    return app
```

The reason is that ```self.app``` is ```None``` in ```init_app()``` and then this will fail:
https://github.com/wichitacode/flask-compress/blob/master/flask_compress.py#L47

Unfortunately, this way of using flask-compress hasn't been covered by a test.