Added a way to zoom to the users location

A added an "zoom-to-my-location" button in the upper-right corner of the map.
When the user clicks that button the geolocation feature of modern browsers is used to zoom to the users location.
By default the user is asked if she wants to share her position, if not the button is deactivated.

I have a working example here: http://mojoaxel.github.io/api.freifunk.net/apps/ffmap/html/map.html