Commit
Add files. PersistentDataStructure.swift defines the abstract class that maintains persistency. Concrete classes implement traversal methods that define the class in order to instantiate the data structure.
WIP commit.
Refactored and conformed to Xcode6-Beta4 (added access modifiers).
Merge branch 'impl-access-modifiers' into impl-persistent-data-structures
Cosmetic changes.
List is implemented and tested, but is inefficiently persistent.
Both List and Tree are implemented and tested, but without persistence.
Commit
The List is currently tail-persistent but fails when a circular reference is created.
The List works in a majority of cases, but fails when there are multiple overlapping appensions.
The List is fully persistent.
Refactored List to be head- and tail-persistent according to the Sleator, Tarjan, et al. algorithm

* impl-persistent-data-structures-refactor:
  The List is fully persistent.
  The List works in a majority of cases, but fails when there are multiple overlapping appensions.
  The List is currently tail-persistent but fails when a circular reference is created.