Change user persistence to use NSUserDefaults

[robinmacharg]

Goal

See #469. This change converts that work to use NSUserDefaults instead of keychain access. It also de-vendors SSKeychain, and reinstates some tests that are now valid.

Review

Outstanding Questions

• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review
  • Release

• The correct target branch has been selected (master for fixes, next for
  features)
• If this is intended for release have all of the pre-release checks been considered?
• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language