Add initial support for scraping Pump.io activity streams

Use symlinks to represent alternate sites with the same structure (i.e.
same server software).

Symlinks are handy and concise, an alternative way would be to introduce
some equivalence mapping, like in the patch below, but I don't really
like that:

  diff --git a/tweeper.php b/tweeper.php
  index a019684..eb12af2 100755
  --- a/tweeper.php
  +++ b/tweeper.php
  @@ -101,9 +101,18 @@ $url = parse_url($src_url);
   if (FALSE === $url || empty($url["host"]))
     die("Invalid url: $url\n");

  -$stylesheet = __DIR__ . "/rss_converter_" . $url["host"] . ".xsl";
  +$equivalence_map = array(
  +  "identi.ca" => "pump.io"
  +);
  +
  +if (array_key_exists($url["host"], $equivalence_map))
  +  $host = $equivalence_map[$url["host"]];
  +else
  +  $host = $url["host"];
  +
  +$stylesheet = __DIR__ . "/rss_converter_" . $host . ".xsl";
   if (FALSE === file_exists($stylesheet))
  -  die("Conversion to RSS not supported: {$url["host"]}\n");
  +  die("Conversion to RSS not supported: {$host}\n");

   $tweeper = new Tweeper($stylesheet);
   echo $tweeper->tweep($src_url);