Feature: Add ability to use multiple <Archive> tags for a source package

You can now use 1 or more <Archive> tags in source packages to download more than 1
tarballs in workDir. This is especially useful for packages like thunderbird where
we're currently creating our own tarball to contain the source code + l10n files.

This will also be useful on some binary drivers which has different tarballs for
every architecture.

pisi will automatically chdir into the directory which matches pkgname-pkgversion. If
no directory matches this, you should provide WorkDir in actions.py as usually.

If you'd like to apply your patches to every subarchive directory, you'll have to
modify the patch levels and set WorkDir to "." (very very very rare case)

Example:

<Source>
  ..
  ..
  ..
  <Archive sha1sum=".." type="tarbz2">http://nvidia.com/driver_x86.tar.bz2</Archive>
  <Archive sha1sum=".." type="tarbz2">http://nvidia.com/driver_x86_64.tar.bz2</Archive>
</Source>

pisi-spec.rng

@@ -48,7 +48,9 @@
                 <!-- FIXME: should be oneOrMore -->
                 <ref name="Description"/>
             </zeroOrMore>
-            <ref name="Archive"/>
+            <oneOrMore>
+                <ref name="Archive"/>
+            </oneOrMore>
             <optional>
                 <ref name="AdditionalFiles"/>
             </optional>