Modified DOPTimelineViewController, to enable initialized in program.

Summary
Add create the loading indicator in DOPTimelineViewController loadView method,
so it is now no need define the indicator in storyboard.
Create DOPTimeLineGestureHandler delegate.

Sumamry:
User can add customized code to handle tap gesture on photos in timeline via implentation The DOPTimeLineGestureHandler#onPhotosDoubleTapped.
Usage:
(1) import DOPTimelineProtocols.h in your code, eg an ViewContrller extends DOPTimelineViewController
(2) implementation onPhotosDoubleTapped, the method will be invoked with the following parameters
    - photoUrls : an array contains photo urls which displayed as an photo boards in timeline.
    - startAt: the index of photo's url which user really tapped.