Improved PushBullet implementation (v2 API, multiple devices, channels)

New: PushBullet supports multiple devices
New: PushBullet channels

Closes #641

src/NzbDrone.Core/Annotations/FieldDefinitionAttribute.cs

@@ -26,6 +26,7 @@ namespace NzbDrone.Core.Annotations
         Checkbox,
         Select,
         Path,
-        Hidden
+        Hidden,
+        Tag
     }
 }