function objMap(obj, func, context) {
  if (!obj) {
    return null;
  }
  var i = 0;
  var ret = {};
  for (var key in obj) {
    if (obj.hasOwnProperty(key)) {
      ret[key] = func.call(context, obj[key], key, i++);
    }
  }
  return ret;
}
For each key/value pair, invokes callback func and constructs a resulting
object which contains, for every key in obj, values that are the result of
of invoking the function:

  func(value, key, iteration)

@param {?object} obj Object to map keys over
@param {function} func Invoked for each key/val pair.
@param {?*} context
@return {?object} Result of mapping or null if obj is falsey