Implement flight mode (applying patches from Tony Espy)
Add persist option to UrfConfig.
daemon: clean up UrfConfig later

... So it makes a bit more sense to keep a reference to UrfConfig in the
arbitrator for persistence.
config: implement persistence via an ini file in /var/lib/urfkill
arbitrator: save/restore per-type killswitch state on startup, dispose.
UrfConfig: apply review from lcp for persistence
UrfArbitrator: apply review from lcp for flight mode
Update default config file for persistence.
arbitrator: set newly found devices to the saved persistent state

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>
Merge pull request #5 from cyphermox/master-switch

Implementation for flight mode, and minimal persistence code to go with it.