Support download tasks

[lilyball]

We should support creation of download tasks. This would probably be another request type that's a sibling of HTTPManagerParseRequest<T>, implemented similarly to HTTPManagerParseRequest except it has a download callback that's invoked when the download is done, instead of a parse callback invoked when the task is complete.

If possible, we should also support canceling with resume state, and re-downloading with that resume state. This probably requires having a subclass of HTTPManagerTask for downloads.

[lilyball]

For anyone looking to do downloads today, you can use the .preparedURLRequest property of HTTPManagerRequest to get an NSURLRequest that you can use with NSURLSession.