Some small fixes to marker detection from last commit. The performance improvements in this branch now seem to be working perfectly.

Modified code to reposition individual marker overlays on hover/focus, instead of repositioning all overlays on map change. Also made a minor fix to button CSS.

Small performance improvements to accessible_gmaps_getMarkerPos() by refactoring code into accessible_gmaps_getMapData

Added lots of functionality and cleaned up code. Ready for cross-browser, cross-application testing.