Download progress

The DownloadInfo interface now exposes Filesize() uint64 and Received() uint64 methods. The value returned by Received() changes in real-time as the file downloads. These are included in the /renter/downloadqueue response, so the client can implement a "download progress bar" by calculating the received/total ratio. This functionality has been added to siac; to watch a file download in real-time, start a download with siac renter download [nickname] [destination] and then run watch -n0.1 siac renter queue.