Add back in redirection for small images using GraphViz library

[mgrdcm]

The original version of II supported a redirect to the Google Chart API for small images. I took out the support because:

1. It wouldn't work with JPG, only PNG
2. I couldn't get the vertical centering to work

However, Google Charts now supports GraphViz chart type which can solve the #2 problem easily. For example:

http://chart.apis.google.com/chart?cht=gv&chs=300x300&chl=digraph{%22hello\nthere%22[shape=plaintext,%20fontsize=10,%20fontcolor=blue];}

There are currently a couple problems:

1. GraphViz is experimental
2. Only supports 'point' sizes for fonts, rather than pixels
3. Colors seem to need to be a name and #hex values aren't working