Updating extension guidelines v2

Continuing #1035 and #1218, #1135 related.
- flaskext.foo is the old thing
- flask_foo is the new thing
- flask.ext.foo is a "fake package" provided by Flask itself so app authors can import their extensions without having to worry which style the extension uses (whether it installs itself as flaskext.foo or flask_foo)
- Nowadays flaskext.foo is not used anymore, so the compat package flask.ext.foo is not needed anymore, everybody should use flask_foo.