Add to_mdc_image_url for generating links to an image server
Call to_mdc_image_url when asked to do so

Still no command-line support yet.
Support composed signs using JSesh syntax
Serve JSesh-generated images from actual production server
Unstack "p:w" in example
Allow user to specify images on the command line
Update example to use -i flag
Bump version to v0.0.3