Add GMGridViewDidScroll, Geometry and pageForContentOffset

[rantav]

After pulling the latest from gmoledina/GMGridView I added a few small features that I needed to complete the picture for invi. I think they can be useful in a broader scope. Please note that the list of commits (12) looks scarry, but most of them are just leftovers from previously non-pulled commits. There are only two interesting commits, https://github.com/rantav/GMGridView/commit/8354e20c78984d746e2c69f6d548af061c3e143f and https://github.com/rantav/GMGridView/commit/664af222282677ace02e1d2e6298ca54b27afb60