Moved Flask-Login into lib/flask_login.py and using flask.ext.login for imports

No new versions or anything but moving the existing Flask extensions; to change the structure:

  * Moved the existing Flask-Login into `main/lib/flask_login.py` (instead of `main/lib/flaskext/login.py`)

  * This includes a file rename to `flask_login.py` compliant with the Flask-Login project naming of the same file (see https://github.com/maxcountryman/flask-login/blob/master/flask_login.py)

For rationale and background see https://github.com/gae-init/gae-init/issues/48; which also describes that/how/why imports should use the `flask.ext.login` notation for Flask-Login (see `main/auth.py` for an example).