Option to exclude specific media directories

[voronind]

It would be useful to skip some media directories from backuping.
E.g. Cache directories

[benjaoming]

That's also a great idea! :)

[ZuluPro]

We need before to list all media files via Django built-in API.
And after there is fnmatch (https://docs.python.org/2/library/fnmatch.html#fnmatch.fnmatch) for match file with wildcard.

We must ensure storages are able to list all files in all directories, files' names must be an absolute path from storage's root. Maybe add a walk function to storages.

[ZuluPro]

Now DBbackup can walk through storage, excludes are coming soon !

[jonathan-s]

covered in #300