Rework, expand and document `BackendBase` interface

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This substantially refactors the base backend interface, adds a number of utility methods, and adds YARD annotations throughout. Notably:

• #add is implemented for cases when a resource should be added, but not retrieved.
• #delete_all is deprecated in favor of #delete_all!
• #empty? is provided.
• #has_resource? is added to check if a resource is in the backend.

The #add and #has_resource methods are generally already implemented as part of #retrieve, so this may be a first step toward a generic implementation of that key method.

Shared examples are added, and tested against a completed Repository implementation. Further work is needed to bring Marmotta and Blazegraph backends up to date.

Some todo items (questions) are added for #get_resource_uri.

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Note that VCR makes this diff enormous.

If i'm doing something wrong with VCR that causes the mass duplication, could someone suggest an alternative approach? If not, are there objections to removing VCR and using simple webmock responses?