Implements Simperium Sync for Notifications

**Notes**
Please, make sure you update your credentials file with Simperium keys.
Background Sync has not been fully implemented in Simperium. It's commented out, for now.

*Thank you!*

**Updated Podfile**
1. Added Simperium dependency
2. Added a custom origin for SRWebSocket. Original repository has a crash, which has been fixed in the mapped fork. A PR has already been submitted, no news from the developer, yet.

**Added Data Model version 17**
1. Updated Note Entity
1.1.	New Attributes simperiumKey / ghostData
1.2.	Nuked payload attribute
1.3.	Nuked icon attribute
1.4.	Nuked account relationship
1.5.	Nuked noteID attribute
1.6.	Updated subject attribute (Transformable + optional)
1.7.	Added body attribute (Transformable + optional)
2.	Updated Account Entity
2.1.	Nuked Notes relationship