fix setup bug with initial map y setting, improve zoom centering

This fixes Issue #27 where resizing makes the map centre jump about - which also causes the map to be somewhat out of line vertically, too.

It also implements a nicer zoom, where the zoom stays centred on the mouse cursor rather than jumping to the middle.  This makes osm-gps-map work more like leaflet.js and Google Maps when you zoom - the thing you're pointing at gets bigger, regardless of where in the window you are pointing.