User geolocation in Google Maps

[stevygee]

• Adds parameter 'mylocation'
• When enabled, geolocation toggle button is shown in map UI
• Adds marker and accuracy circle to map
• Watches user position, re-centers map
• When dragged, position keeps updating, but map no longer follows
• Geolocation can be turned off
• Fixes AJAX markers not updating when changing center only

Fixes #98