Add support for conditional get of metadata files.

This patch adds support for only updating metadata files that have
changed on the server. This reduces bandwidth used, and also allows
us to skip metadata parsing, which speeds things up significantly.

Thanks to Dyonisius Visser for implementing this!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2980 44740490-163a-0410-bde0-09ae8108e29a