Added jpgraph functionality to display pie charts for responses. Submitted by Pieterjan Heyse. Requires jpgraph class to be installed and working. Disabled in config.php by default. Some bugs to iron out.

git-svn-id: file:///Users/Shitiz/Downloads/lssvn/trunk/unstable@861 b72ed6b6-b9f8-46b5-92b4-906544132732