Url for with parameters

Needed so we can do:

```ruby
AWS::S3::S3Object.url_for 'path/to/file', 'assets', :expires => 100,
  :query => {'response-content-disposition' => 'attachment; filename=foo.zip'}

# use it in any of these:
AWS::S3::S3Object#url(:query => {})
AWS::S3::S3Object.url_for(name, bucket, :query => {})
AWS::S3::S3Object.path!(bucket, name, :query => {})
```