Rework the way IDF's SCM interface provides downloadable snapshots.

Instead of returning a command which gets executed and which should
pass through / stream its output data to the client, we're just
returning an instance of Pluf_HTTP_Response. This is needed, because
some SCMs, most noticable monotone, have no locally executable command
to provide a snapshot archive (and probably never will for our kind
of setup).

We therefor added a little BSD-licensed class "ZipArchive" which allows
the creation of pkzip-compatible archives on the fly by letting it eat
the file contents directly feed from the (remote) stdio instance.
Download performance is ok and lies between 15K/s and 110K/s, but at
least we do no longer block the browser while we pre-generate the zip
file server-side.

Thanks to Patrick Georgi for all his work!

src/IDF/templates/idf/source/mtn/tree.html

@@ -48,7 +48,7 @@
 </table>
 {aurl 'url', 'IDF_Views_Source::download', array($project.shortname, $commit)}
 <p class="right soft">
-{* <a href="{$url}"><img style="vertical-align: text-bottom;" src="{media '/idf/img/package-grey.png'}" alt="{trans 'Archive'}" align="bottom" /></a> <a href="{$url}">{trans 'Download this version'}</a> {trans 'or'} *}
+<a href="{$url}"><img style="vertical-align: text-bottom;" src="{media '/idf/img/package-grey.png'}" alt="{trans 'Archive'}" align="bottom" /></a> <a href="{$url}">{trans 'Download this version'}</a> {trans 'or'}
 <kbd>mtn clone {$project.getSourceAccessUrl($user, $commit)}</kbd> <a href="{url 'IDF_Views_Source::help', array($project.shortname)}"><img style="vertical-align: text-bottom;" src="{media '/idf/img/help.png'}" alt="{trans 'Help'}" /></a>
 </p>