Add `patch` method

The patch method should operate like save, except it should pre-load and compute the optimal diff for saving:

var diff = function(before, after) {
  var patch = after;

  if (angular.isArray(after)) {
    patch = [];
  } else if (angular.isObject(after)) {
    patch = {};
  }

  if (angular.isObject(before) && angular.isDefined(after)) {
    // Compute difference for changed properties
    angular.forEach(before, function(left, key) {
      if (!angular.isDefined(after[key])) {
        return false;
      }

      var right = after[key];

      if (angular.isArray(after) || !angular.equals(left, right) || key === 'id') {
        patch[key] = diff(left, right);
      }
    });

    // Add new properties to patch
    angular.forEach(after, function(right, key) {
      if (!angular.isDefined(before[key])) {
        patch[key] = right;
      }
    });
  }

  return patch;
};

The save method should no longer pre-fetch a deep tree, and only patch should do that for calculation only, then defer to save for the actual saving.

Start by locating the existing save and model/API flow in the repository. Compare how save currently pre-fetches the deep tree with the requested patch behavior, then verify that patch computes the supplied diff and delegates the actual save. Done means patch performs the calculation while save no longer performs that pre-fetch.