A couple of tweaks:

  * encode/decode URIs correctly; URIs are Unicode and
    Data.ByteString.Char8.[un]pack will mangle them.

  * turn certificate verification off on Mac and Windows rather than just
    bombing out if we try to use SSL there

  * shorten `mappend` to <>

  * stop calling "withOpenSSL" every time we download a URI; not only is it
    expensive, but I don't think it's safe to call SSL_library_init() multiple
    times in a multi-threaded context. Instead put a warning message in the
    docs asking the user to run it in main.