Hide/show image view during transition

Hides image view before zoom in transition and shows it after zoom out
transition.
Single tap to zoom out. Single tap or drag to dismiss.

Instead of single tap to dismiss while zoomed in, single tap will zoom
back out.
When zoomed out, single tap or draggin will dismiss. Facebook style.
Scroll view settings moved to xib
Merge pull request #3 from brianeunilkim/master

Single tap to zoom out. Single tap or drag to dismiss.