Added raster images

Can now upload bitmap images - only tested with png, jpg, and gif. Animated .gifs seem to bug out the server sometimes and don't really animate on the client (they may change to a random frame if you select them)... it's not recommended to upload animated .gifs at this time.

Images can be uploaded by drag and drop or from clicking the image upload button for a traditional file chooser.

This addition requires a new version of the paper node module to be installed. Please delete the directory from the node_modules directory and then ```npm install paper``` again. This is important as the server will crash otherwise if someone uploads an image!

This addresses issue #62.