Enable Distance Filtering in Drupal Services

When developing mobile apps, location is one of core components that nearly all mobile apps are developed with.  However, when it comes to building mobile apps with Drupal, working with location can be challenging.  There are number of geo-related modules and discussions on enabling location by working with Views.  But in our case, we decided not to use Views, but just with the Services modules, to provide the maximum performance to mobile apps.