Gets the list of all the pids of the system.
@param  {Function} callback Called when the list is ready.
function get(callback) {
  if (file === undefined) {
    callback(
      new Error(
        os.platform() +
          ' is not supported yet, please open an issue (https://github.com/simonepri/pidtree)'
      )
    );
  }

  var list = require('./' + file);
  list(callback);
}