Basic code and tests for demodb manage support.

- Still need tests for edge cases & multi-db support.
Specify the header for demostorage hooks instead of just a prefix.

- Also add tests for edge-cases.
Merge branch 'master' into satchit-demostorage-hooks
Refactor (to simplify code) based on review.

- Also use "public" attribute for accessing storage.
add note about Jim's suggestion re:demostorage hooks
Merge pull request #2 from zopefoundation/satchit-demostorage-hooks

Provide hooks to enable push/pop on underlying demostorage