Adds the following mappings for the exif helper:
* GST_TAG_CAPTURING_DIGITAL_ZOOM_RATIO -> DigitalZoomRatio
* GST_TAG_CAPTURING_FOCAL_LENGTH -> FocalLength
* GST_TAG_CAPTURING_SHUTTER_SPEED -> ExposureTime, ShutterSpeedValue
* GST_TAG_CAPTURING_FOCAL_RATIO -> FNumber, ApertureValue
* GST_TAG_CAPTURING_ISO_SPEED -> ISOSpeed, PhotographicSensitivity

Tests included.