Add frontpage micrograph support

Apologies for not submitting this through your preferred method previously.

This adds a "micrographs" frontpage function similar to the minigraphs, but with the difference that multiple sections are allowed, each corresponding to a row in the $config['frontpage']['micrographs'] array.

Here's an example config excerpt:

...
$config['frontpage']['order'] = array('device_status_boxes', 'minigraphs', 'micrographs', 'status_summary', 'eventlog');
...
$config['frontpage']['micrographs']['network']['ids'] = '1,device_bits,router1;2,device_bits,router2;3,device_bits,switch1;4,device_bits,firewall1;5,device_bits,firewall2';
$config['frontpage']['micrographs']['network']['title'] = 'Network';
$config['frontpage']['micrographs']['servers']['ids'] = '6,device_bits,vmhost1;7,device_bits,observium;8,device_bits,fileserver;9,device_bits,vmhost2;10,device_bits,backup';
$config['frontpage']['micrographs']['servers']['title'] = 'Servers';
...

This produces the attached output on the front page.

Both the row labels and the graph labels are optional - if you leave them both out you get just a bare row of clickable graphs. There's no label justification, which looks a bit funny when labels are different lengths, but i figure people can just add space or whatever to make their front page look how they want it.