Added code to map urls to file-names so that jss-scipt can open an editable file instead of just a buffer with a dump of data from the server.
Don't try and remove a text property we never set in jss-script buffers (this prevents emacs from thinking the buffers have been modified)
Added note mentioning that jss won't check if the locally opened file and the code the server saw are the same. Also added a prefix argument to jss-frame-goto-source to force loading the code from the server.