Added Concern and Metadata classes

- Refactored Sluggable specs to use predefined models.
- Current spec status: 145 examples, 0 failures.
Refactor Concern into Module instead of Class.

- Current spec status: 145 examples, 0 failures.
Convert Sluggable concern to new Concern architecture.

- Refactor stored Concern to use relations.sleeping_king_studios instead of
  string prefix in hash key. This avoids Mongoid trying to load non-Mongoid
  relations that were stored in the base-level #relations hash.
- Current spec status: 170 examples, 0 failures.
Add documentation for new Concern, Metadata, revised Sluggable

- Current spec status: 170 examples, 0 failures.
Minor internal refactor into a Sluggable::apply method

- Current spec status: 170 examples, 0 failures.
Major internal refactor to HasTree
Add dependent => destroy spec for Villain model
Convert specs for HasTree::CacheAncestry

- Current spec status: 262 examples, 0 failures, 1 pending.
Refactor HasTree::CacheAncestry to Concern::apply pattern

- Current spec status: 300 examples, 0 failures.
- Madness? THIS! IS! SLEEPING KING STUDIOS!
Incremented version to 0.6.0 for release.

- Current spec status: 300 examples, 0 failures.
- Minor documentation changes.