Simplified config take 2

[iromli]

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/default_config.py.

2. Use default config and override necessary values using user-defined config file.

   Default config:

   # users/default_config.py
   MAIL_SUPPRESS_SEND=False
   DEBUG = False

   Custom config:

   # /path/to/custom_config.py
   MAIL_SUPPRESS_SEND=True
   DEBUG = True

   Example usage:

   USERS_CONFIG=/path/to/custom_config.py python runserver.py

3. Use default config and override necessary values using environment variable.

   Default config:

   # users/default_config.py
   MAIL_SUPPRESS_SEND=False
   DEBUG = False

   By using enviroment variable, we could override specific item, for example:

   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 controls on how to configure app in various usecases, e.g. Travis CI integration.

[akbargumbira]

Thanks @iromli, this is very nice..