Simplified config take 2

This changeset introduces [flask-appconfig][] integration to manage app configuration loading. By using this extension, we have more possibilities to configure the app:

1. Use default config `users/config.py`.

2. Use default config and override necessary values using user-defined config file.

    Default config:

    ```python
    # users/config.py
    MAIL_SUPPRESS_SEND=False
    DEBUG = False
    ```

   Custom config:

    ```python
    # /path/to/custom_config.py
    MAIL_SUPPRESS_SEND=True
    DEBUG = True
    ```

    Example usage:

    ```sh
    USERS_CONFIG=/path/to/custom_config.py python runserver.py
    ```

3. Use default config and override necessary values using environment variable.

    Default config:

    ```python
    # users/config.py
    MAIL_SUPPRESS_SEND=False
    DEBUG = False
    ```

    By using enviroment variable, we could override specific item, for example:

    ```sh
    USERS_MAIL_SUPPRESS_SEND=True USERS_DEBUG=True ./runtests.sh
    ```

    Note, to use the environment variable properly, the `USERS_` prefix must exist.

Now we have more control on how to configure app in various usecases, e.g. Travis CI integration and so on.

[flask-appconfig]: https://github.com/mbr/flask-appconfig