Application context

Hi,

One of the error message regarding application context is a bit confusing (here: https://github.com/miracle2k/flask-assets/blob/master/src/flask_assets.py#L282), you're asking for an "application context" whereas what you want is a "request context" (at the time you wrote this I guess application context didn't exist).

I wrote a small PR to also check for an app context.

Basically we are now able to do:

    with app.app_context():
        cmdenv = CommandLineEnvironment(environment, log)
        cmdenv.build()

(It's my first PR to an open source project btw :))