fix 1 bug and add 1 animation to improve user experience.

fix bug: if picture focused is smaller than view’s bounds, it can’t be zoomed in.
how to reproduce this bug: in the method "- (NSURL *)mediaFocusManager:(ASMediaFocusManager *)mediaFocusManager mediaURLForView:(UIView *)view" of MainViewController, return a url of small picture.

add 1 animation: same test procedure of the first bug. If return a url of a small picture, the original animation is not smooth at the last step. The new code provides better animation.