* added a new 'by_path' function to be used for the UPLOAD_TO callback
* change move_files_to_folder so that it causes a storage backend move as well. This is useful when the on storage path depends on the models.foldermodels.Folder the file is uploaded to
Added a setting for stating whether UPLOAD_TO callack depends on the folder where the file is ment to be uploaded
Fixed issue with files moved to clipboard (they were uploaded to filer.models.virtualitems.UnfiledImages.name)
Validate that no folder that might overlap with the upload/clipbloard folder might get created.
* moved file moving logic to filemodels.File
* added unique file name/folder constraint
* s/LOGIC_PATH_EQ_ACTUAL/FOLDER_AFFECTS_URL/; it just sounds better
* removed unused imports
Added a 'last line of defence' check that ensures that no duplicate files can exist in the same fodler
Fix various issues regarding the enforcing of file unicity

* moved all clipboard unicity relatd validation in models.tools (there's still a certain amount of duplication, but I'll remove that in a future commit)
* removed move and copy actions; this would cause serious problems if recursivly moving/copying folders: if during moving/copying an exception is raised (ex due to connection problems with the storage backend, then we end up with an inconsitent folder hierarchy)
* removed _move_file_to_new_location and use the already existing _copy_file method
* only delete the previous version of the file AFTER the new version of the object has been commited to the db. This ensures that we'll won't end up having filer file objects pointing to non-existent files
* remove discarding feature and replace it with delete
Fixed broken unit tests.
Added test for checking that when FOLDER_AFFECTS_URL is set file names in the clipboard are unique
Also added a context manager for overriding filer settings in tests.
Read FOLDER_AFFECTS_URL from the corresponding project settings
Validate folder name uniqueness on folder renames on folder moves/copying.
Fix for #304: make 'copy files and/or folders' work when the current folder is the root folder.
Place the unique name constraint on both files and folders.
Added a test for pasting from clipboard to a folder and checking that duplicate file names are not allowed.
Moved the SettingsOverride helper class to tests/helpers.py
Make sure the error message on duplicate folders works with unicode characters as well.
Made all error messages in case of duplicate files/folders cope with unicode characters.
If a file didn't get pasted from the clipboard, delete it.

If the file wouldn't get deleted it would remain in an orphan state: attached to no folder and to no clipboard.
Enforce unique file and folder constraint regradless of whether FOLDER_AFFECTS_URL is true or not