StorageDownloadView deprecates PathDownloadView

[benoitbryon]

Currently (version 1.2), StorageDownloadView inherits PathDownloadView.
Should be: StorageDownloadView makes it easy to PathDownloadView.

• Add get_storage(), get_storage_class and get_storage_kwargs() methods to StorageDownloadView, i.e. make the storage overridable.
• By default, get_storage() uses Django's get_storage_class(settings.DEFAULT_STORAGE).
• In documentation, explain that (deprecated) PathDownloadView is a StorageDownloadView with storage=FileSystemStorage(location=CUSTOM_LOCATION)
• Deprecate PathDownloadView.
• Optionally setup a backward-compatible PathDownloadView which inherits StorageDownloadView and uses FileSystemStorage as a default.