GPS location should update continuously

[jwalgran]

The initial implementation stops tracking location as soon as it is found

https://github.com/OpenTreeMap/otm-ios/blob/59a0c7cc7abd3ce6e78b250b4c48ef070f80846d/OpenTreeMap/src/OTM/Controllers/OTMMapViewController.m#L900

This prevents the battery drain cause by continuous GPS updates, but ends up causing confusion since the dot does not move as the user walks around.

• Read up on the location services documentation and update the GPS code to continuously update the location dot when the app is in the foreground. We are not routing, so we do not need to update in the background. https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/LocationAwarenessPG/CoreLocation/CoreLocation.html#//apple_ref/doc/uid/TP40009497-CH2-SW1
• See if we can replace our custom location dot with the built in dot that is part of the MapKit APIs.

[jwalgran]

I have implemented this as part of #319