* Add a @requires_admin decorator
It can be used to protect specific endpoints with ADMIN_PASSWORD
(a password that is stored unencrypted in the settings)
The decorator has no effect if ADMIN_PASSWORD is an empty string (default value)
* Require admin permissions to access create project endpoint
When ADMIN_PASSWORD is not empty, project creation form on the
home page will be replaced by a link to the create project endpoint
so one is able to enter the admin password before filling the form
Default settings from app's root path are loaded first
Settings are then overriden by /etc/ihatemoney/ihatemoney.cfg
or by another file which path is set in an env var
Fixes#187
