Add an entry for Save as Image plug in

Allow users save the current chart as an image file to local disk. 
Screen shot here: http://zizhujy.com/blog/post/2013/07/02/A-Flot-plugin-for-saving-canvas-image-to-local-disk.aspx
Merge pull request #8 from Jeff-Tian/master

Add a plug in to allow users save the current chart as an image file.