Added cache clearing & setting cachePrefix for Aws S3

Not sure if this is the best solution, but I used a setter to get the cachePrefix to fix #269 & #251.
This would require adding the prefix to the resolver like;

```yaml
acme.imagine.cache.resolver.amazon_s3:
        class: Liip\ImagineBundle\Imagine\Cache\Resolver\AwsS3Resolver
        arguments:
            - "@acme.amazon_s3"
            - "%amazon_s3.bucket%"
        tags:
            - { name: 'liip_imagine.cache.resolver', resolver: 'amazon_s3' }
        calls:
            - [setCachePrefix, ["%liip_imagine.cache_prefix%"]]
```

Anyway, it clears all files inside the predefined directory in your bucket when running the `app/console cache:clear` command.