Don't download Tor binaries if they already exist

This branch changes the logic for downloading and verifying the Tor binaries. Currently we check the modification time of each file on the server, download the file if the server's copy is newer than our copy, then verify the hash. This requires a network connection even if the file has already been downloaded. This branch only downloads the file if it doesn't exist. This makes it possible to build without a network connection after the initial download. However, it means that each time we upgrade the Tor binaries, verification will fail until the local copies are deleted.