Removing unused object in Setting model.

- Removed "import cache_setting"
Upgraded the Directory.logo attribute with more features

- Resizing and cropping are available on the directory logo

- Applied the same as the file module
Removing extra code in directories

- This code would associate a logo with it's directory.  All of this logic is not handled by the File model

- The media server logic is also handled by the File model now
Updating the Directory.logo attribute

- Only returning the logo_file FileField attribute if a logo is available; else None type object.
Enabling the Directory.logo clear checkbox to work on forms

- Because we're using the File model, extra logic has to be added to clear the directory logo when the clear box is checked.