Make unhandled hosts/services configurable

This patch moves the definition of unhandled/open problems
into settings.py

As a side effect of this, definition of unhandled can be overwritten
in adagios.conf by adding these to the config file:

UNHANDLED_SERVICES = {
    'state__isnot': 0,
    'acknowledged': 0,
    'scheduled_downtime_depth': 0,
    'host_state': 0,
    'host_scheduled_downtime_depth': 0,
    'host_acknowledged': 0,
}
UNHANDLED_HOSTS = {
    'state': 1,
    'acknowledged': 0,
    'scheduled_downtime_depth': 0
}

This partially fixes #471 although we are not providing the ability
yet to configure this from the settings web interface.