Enable end users to upload files through the Request resource

Hi,

We are integrating our web interface with Zendesk and I didn't find a way to include files in a comment using the Request resource as an end user. I modified the Comment class to include this functionality, I don't know if I'm missing anything or if this is the right way to achieve this.

With this change I can upload files as follows:

```ruby
comment = ZendeskAPI::Request::Comment.new(client, {
  :value => "My logs"
  })
comment.uploads << "/path/to/the/logs"
comment.save!

request.comment = comment
request.save!
```