In FileBackendBase/FileBackend:

In FileBackendBase/FileBackend:
* Moved some public static functions from FileBackend to FileBackendBase as the later defines the public API.
* Made splitStoragePath() return null if the backend or container name is empty.
* Made normalizeContainerPath() kill leading directory separators.
* Added more unit tests and made some documentation tweaks.
In FSFileBackend:
* Added resolveContainerName() to disallow '.' a container name, since this would cause a traversal.