Remove EnvironStorage from Environ interface

Storage is now optional; backups will fail
if the environment does not support provider
storage. None of the providers' storage
implementations has been taken away yet, so
this is not a problem in practice.
Update tests to not use Environ.Storage()
Add TODO about migrating backups
Add comment to environs.LegacyStorage
Address review comments

Reinstate some tests, fix comments, improve error message
Merge pull request #825 from axw/environs-remove-storage

Remove EnvironStorage from Environ interface

Storage is now optional; backups will fail if the environment does not
support provider storage. None of the providers' storage implementations
have been taken away yet, so this is not a problem in practice (yet).