Add Configurable Upload directories via settings

I dont know if you want any of this but I figured I'd share it. I did it for a specific need we had. 

Basically give the user a way to upload to different predetermined paths.

Ex Config in settings.py :
```
UPLOAD_DIRECTORIES = (
    ('path1', 'Path1'),
    ('path2', 'Path2'),
)
```
Those end up as options when adding a new file.

![screenshot from 2013-10-16 11 48 20](https://f.cloud.github.com/assets/372217/1343988/ad7da1de-367a-11e3-91de-fb8d97d4c7b5.png)

Im happy to flesh it out more if you'd like.