Unique/Name Key

[willronchetti]

• Provide keys.rst - an overview of how the various keys work in Snovault and how it applies to FF/CGAP. This should be useful in making decisions with respect to the data model
• Rename unique_key on the collection decorator to traversal_key. The reason being that while the traversal_key must be a unique key calling it unique_key is misleading as it is different than marking an item property as a uniqueKey on the item schema.
• Clarify that name_key only augments the resource path and must match the traversal_key if it is used.