Improves Email Demos (#344)

* Observer Crash

• Fixes an issue where, in rare cases, users who call OneSignal.removeSubscriptionObserver() inside of the onOSSubscriptionChanged() delegate method would cause a crash because the observers array gets mutated while it is being enumerated.
• This should have been a relatively rare crash since the delegate selector gets offloaded to the main thread which generally would introduce enough of a delay to prevent the crash.

* Fix Null Crash

• Fixes a crash highlighted in #343 where nil userId is inserted into dictionary
• Changed to better syntax instead of a category on NSNull

* Add Email to Demos

• Adds email to both the Swift and Objective-C Demos

* Add Missing Files

• The previous commit was missing some project class files & launch screen storyboard.

* Add Icons to Swift Demo

• Adds OneSignal icons to Swift demo

* Fix getPermissionSubscriptionState() Bug

• getPermissionSubscriptionState() was not properly returning email subscription state, which was a contributing factor in bug #89 in the Unity SDK repo.

Author: Nightsd01