Add option to output an array of image hash uris

[calum]

Instead of accepting a file, it could accept a URL to an image, and then outputs an array of uris representing pieces. This avoids the need to store the images locally.

[calum]

This may work already. Similarly to #22. Needs to be checked though

[calum]

can be done:

Data URI

A Base64 data URI can be generated in the same way as a Buffer, using:

image.getBase64( mime, cb ); // Node-style callback will be fired with result

[calum]

this can be used to create a drag-and-drop jigsaw web application to demonstrate jigsaw-builder