Fixes in GRKPickerViewController to present in a popover

method
-presentInPopoverFromBarButtonItem:permittedArrowDirections:animated:
now uses the given parameter
Added method
presentInPopoverFromRect:inView:permittedArrowDirections:animated:
Typo in comments

if -> is
added methods picker:shouldSelectPhoto: and picker:shouldDeselectPhoto on GRKPickerViewControllerDelegate + impacts
Changes on GRKPhoto

_ added methods firstImageWithWidthGreaterThan: and
firstImageWithHeightGreaterThan:
_ added a cache on imagesSortedByWidth and imageSortedByHeight
Update in ReadMe

added section "Applications using GrabKit"
Update in .gitignore

ignores subdirectory for submodules