Make use of flask.current_app context local

Flask extension development guide

(http://flask.pocoo.org/docs/extensiondev/#the-extension-code)

suggests using current_app instead of storing app on self during initialisation.

Here's citation:

   Note on ``init_app``

   As you noticed, ``init_app`` does not assign ``app`` to ``self``.  This
   is intentional!  Class based Flask extensions must only store the
   application on the object when the application was passed to the
   constructor.  This tells the extension: I am not interested in using
   multiple applications.

   When the extension needs to find the current application and it does
   not have a reference to it, it must either use the
   :data:`~flask.current_app` context local or change the API in a way
   that you can pass the application explicitly.