Make map automatically follow a user's location

[dsummersl]

During our alpha-phase we'd like to walk the neighborhood, and it would be very helpful if the map automatically updated to show drains around the user.

Things to not break:

• if a person uses the address search feature, it should continue to work! If you search for an address, the map should change to that address...and not change again unless the website viewer has moved.
• if a drags the map around then the map should stay where the user left it, until their location changes.
• if the viewer is not IN durham, then don't update the map to their location.

[specious]

A toggle switch to turn on "follow GPS" mode would be a handy feature.