Reuse previously added images

Hello, I added in support for reusing images that have already been added into the PDF, per issue https://github.com/MrRio/jsPDF/issues/99. I tried to follow the style conventions I noticed in the plugin already, but if I've done anything you don't like, feel free to clean up or tell me how to clean it up. :)

The new functionality can be used in the way suggested by n8v:

doc.addImage(imgData, 'JPEG', 15, 40, 180, 180, 'Octonyan');
doc.addImage('Octonyan', 'JPEG', 15, 400, 180, 180);