Change preview file to and README to better accommodate running on a rem...

...ote server

This PR edits the preview.html file to be more generic about the host's IP so that it's easier for folks running this on remote servers (which, given DataMaps' requirements is probably likely to be common); it also edits the README to reflect the (simplified) preview instructions.

I figured since I needed to do this after getting it all set up on a remote box, I might as well contribute it back. Let me know if you have any concerns or questions!