add TorHttpDownloader for handling repos on Tor Hidden Services
Tor Hidden Services are on domain names that always end in .onion, so there
is a URL pattern matcher that chooses which Downloader subclass to use
based on testing for .onion. This is a quick, dumb implementation. It
does make any attempt to see if Tor is running or even installed. That
can come once NetCipher is easy to handle in the context of FDroid.