Extended Fog::Storage::AWS::Files files method

This method now accepts an hash of options that is fed to the Fog::Storage::AWS::Files constructor.

It's useful to filter the files that are returned prior to actually getting all the files.

Example:

```ruby
Fog::Storage::AWS::Directory.files(
  :directory => some_bucket, 
  :service => some_bucket.service, 
  :prefix => 'this/is/some/key/prefix'
)
```