Add changelog for #1218
Drop Extension==dev requirement

pip doesn't install links included in the description of projects
anymore. Therefore ==dev install doesn't work anymore.
Drop Python 2.6 minimum requirement for extensions

Python 2.6 is not supported by python-dev anymore and does not get any
security updates. Even though Flask supports 2.6 at the moment, I think
it's not necessary for any extensions that are going to be approved in
the future to support 2.6.
Require Python 3.3 and higher for extensions

Flask and several extensions already supports Python 3.3 and higher. By
requiring approved extensions to support Python 3.3 as well we can
quickly achieve better Python 3 adoption and make using Python 3 easier
for users.

The effort of supporting both Python 2.7 and Python 3.3 is small enough
that it shouldn't be a problem to require this from extension authors.
Don't allow namespace packages anymore
Fix wording