Enhancement block storage controller

First PR in the series
- establishes the structure within the code-base
- adds the controller and model, but no functionality
- controller is in read-only mode with PUT returning 405 (Method Not Allowed)
- all other end-points return 501 (Not Implemented) for the time-being (they are all TODO)
- all tests pass, 100% code-coverage
head_block() generates the exception, the rest is stubbed out for now.