Add support for setting custom headers for files uploaded to S3

Today there's only support for setting the ``Cache-Control`` header. I've removed the ``S3_CACHE_CONTROL`` setting and added ``S3_HEADERS`` which supports setting whatever header you want:


```python
S3_HEADERS = {
    'Expires': 'Thu, 15 Apr 2010 20:00:00 GMT',
    'Cache-Control': 'max-age=86400',
}
```