fix for flexget sending duplicate pushes with pushbullet

issue described here:
http://discuss.flexget.com/t/pushbullet-plugin-sending-2-pushes-per-match/81
properly fixed the multi device push duplication

by not specifying any device idens, the push will be sent to all devices
of the user. or the user can specify only the devices they want to
receive pushes on.
minor update to pushbullet plugin
Merge pull request #253 from dj-nitehawk/develop

fix duplicate pushes with pushbullet