Adapt to new Flask extension module schema

Naming your Flask extension module "flaskext.foo" is now discouraged in
favor of naming it "flask_foo". To accommodate both styles, Flask 0.8
and higher allows you to import "flask.ext.foo", which will first try
"flask_foo" and then try "flaskext.foo". We should do this for all our
flask extension imports.

In the short term, this allows sagenb to build against the latest
version of Flask-AutoIndex, i.e. it deals with the following commit:
https://github.com/sublee/flask-autoindex/commit/2138855d69a124cdaad