fix map loading, add remove() method, minor fixes

* Initialize the map on panorama 'ready'
  - on slow bandwidth - tested using apache2 module ratelimit - the map div
    was set to 'position:relative' and displayed above the panorama
  - fix also the "double-click on marker to switch panorama" issue

* added map.remove()

* map.hide() toggle the map visibility

* show() instantiate map if needed, else toggle map visibility

* moved map.callback() definition, from map instantiation (in map_panorama_init()) to Map.prototype.callback
example/js/main.js: set map active=true