Support for graph again (using jqplot)

This is a re-implementation of the puppet activity graphs, this time based on jqplot which is not dependant on the rails3 deprecated javascript libraries.

A live demo is available here: http://ec2-107-22-23-153.compute-1.amazonaws.com:3000/nodes/1 (may be removed in a few days)

The new graphs look slightly different from the old ones but I think it's an acceptable replacement.