Add failing tests to demonstrate problems with persistence

localStorage and sessionStorage persist across tabs and windows.
depot.js stores currently break this expectation because their
internal id caches are not kept up-to-date across multiple
instances (i.e., multiple tabs/windows) when records are added.
Add .refresh() method to keep stores updated across tabs/instances

New method checks the store object's cached `store` string against
the latest actual value in the storage adapter, and overwrites the
cached `ids` array with the updated array if there's a difference.

The `add` method now updates the `store` string in addition to the
`ids` array.
Move .refresh() calls below initial var declarations for consistency
Merge pull request #8 from jdbartlett/tab-safe

Fix persistence across multiple tabs / windows / instances