Allow to update field values without saving.

Using an AJAX call we can obtain geolocalized data without saving the model.
The admin add a 'geo/' URL to call.
Show map also on first editing and set configurable defaults using
its own settings file.
Update README.
Set a configurable default for geographical coordinates.
Modify widget so to be more suitable for inlines.
Add example models.
Add templatetag {% easy_maps_js %} to render <script> for google maps.

It uses the template 'easy_maps/map_js.html'. Also modify the admin
template to include it when necessary.
easy_map tag now display more than one marker.
Remove useless fields.
Remove useless dictionary entry.
Use relative imports.