Put the javascript files into the Media class.

In this way is avoid the multiple inclusion of the Google API
javascript file.
easy_map tag now display more than one marker.
Remove single marker JS code.
Remove useless fields.
Remove useless dictionary entry.
Use relative imports.
Factorize form definition.
Move callback function to the button.
Extends change list admin template to show saved addresses on the map.

Also modify the Media class.