Allow acl's other than public-read by adding more info to S3DIRECT_DESTINATIONS

Firstly thanks for this package, it's a real timesaver : ) 

# Motivation

I'm working on a private project that is moving from a dedicated box to Heroku and the project handles a number of large (50mb+ files) that **have** to be private and not accessible to the public. This package done everything **except** allowing to configure acl's other than 'public-read' in `settings.S3DIRECT_DESTINATIONS`.

# What I've changed

1. Added ability to set a canned acl policy for uploaded files.

2. Documented the behaviour in README.md

3. Finally, removed two unused imports, uuid from utils and permission
denied from views.

# How it works
Quite simple and I've tried to be as un-invasive as possible. I've added ability to add an extra element into the tuple for an acl.

I'd love to see this feature merged in and if my workings aren't to your satisfaction, please let me know what I can do to get this into master and hopefully get a release out.

Cheers, 

Jamie.