Correct the order of suggested syntax for extension imports

According to https://github.com/mitsuhiko/flask/issues/1092#issuecomment-47118613 and https://github.com/mitsuhiko/flask/pull/1085#issuecomment-45466907 , the correct order to attempt to import extensions should be:
1) flask_foo
2) flask.ext.foo
3) flaskext_foo.