Invert the settings import layout

This change isn't necessary for an example but I've found it a better
convention to start using before a project grows: 

This changes the model from settings importing local_settings to have
the environment-named settings files (settings_prod, etc.) import the
common settings first and then apply local customizations so the local
settings can easily override anything without worrying about import
loops.  

This also tweaked the local settings structure slightly to base the path
on the file location rather than the current directory so it works if
someone changes directories before starting the django development
server.