Add pool graphs to the status page

The commit adds pool size graphs to the server status page for privileged users.
RRDtool is required to store pool size statistics and show graphs.
If the RRDtool is not installed, status page looks as usual without graphs.
Updates of the graphs occurs during 'backuppc nightly' runs.

This patch had originally written by Ludovic Drolez and Nicholas Hall:
http://sourceforge.net/mailarchive/forum.php?thread_name=20080302014621.GG618%40renesys.com&forum_name=backuppc-users
http://www.mail-archive.com/backuppc-users@lists.sourceforge.net/msg09451.html