fixed import error
LUN-1256: files & folders can be soft-deleted
* file's fk to folder needs to be lazy loaded, otherwise folder
model will not be aware of the 'all_files' related name(caused by
the fact it will get loaded before File model)
LUN-1256: added deleted_at field in order to track trashed objects
LUN-1256: added mixin for common Querysets methods + fixed tests
LUN-1256: * added custom managers for trashed/alive/all items.
Manager 'objects' will now refer to alive objects.
* Tree corruption checker should use the tree manager
* remove tox log mistakenly added
LUN-1256: soft delete makes actual files to be moved to _trash/
LUN-1256: added admin view for trashed items
* fixed migration to use south's orm model manager
LUN-1256: added views for trashed filer items
LUN-1256: pytz is now a requirement for django filer
LUN-1256: + pep8 imports;+ trash prefix for settings;+ docstrings;
LUN-1256: fixed import error for filer_settings
Merge pull request #34 from pbs/LUN-1256

Lun 1256