[Vision] add a new function 'detect

[Vision] add a new function 'detect_all' in image.py

There is a function detect in image.py for detecting multiple feature types. Could we add a function called detect_all to detect all feature types at one time? This is my proposal:

    def detect_all(self, limit=10):
        """Detect multiple feature types.

        :type limit: int
        :param limit: The number of faces to try and detect.

        :rtype: list
        :returns: List of
                  :class:`~google.cloud.vision.entity.EntityAnnotation`.
        """

        features = [Feature(x, limit) for x in FeatureTypes.__dict__.keys() if x[:1] != '_']
        annotations = self.detect(features)
        return annotations[0]