Updates to INSTALL and map-poller.php

Updated INSTALL instructions to provide full install instructions.

Updated map-poller.php to include a $weathermap_url variable to ensure
images are correctly generated when map-poller.php is called.

Updated weathermap to accept —base-href from map-poller.php to set the
base href in the html output.

updated changes.diff file to show changes from original weathermap to
this.