Fixes regarding IDs and cancelling

[HernanZh]

Local notifications have some problems because IDs are not unique and it's impossible to cancel them between sessions in iOS.

• The IDs of the notifications always started at 0 between sessions, this is now Date.now() making it more unique.
• The iOS plugin keeps a local dictionary of notifications, but it's not persistent. The scheduled notifications are now loaded into this dictionary during initialization.