Add support to flask extension and Babel

I'm using this fork on my flask.ext like this::

    from flask.ext.appbuilder import translations

    self.babel = Babel(app, translations)


flask-babel will merge the package translations with the app translations (if one exists). It would be great if you accept this on your project because i am already using it, and would preffer to use your "oficial" extension (has a requirement to my package) then my fork.