@jwilling => add support for consistent margins on items in the grid layout

I've been happy with JNWCollectionView! 

So I'm building a grid view but I prefer to have a consistent margin on the items. As there's already a vertical spacing modifier I've added one for horizontal. 

I'll probably send another PR to deal with alignment of the grid view items ( left / center / right ) so I can finish up the style I'm looking for, if you have any particular direction you want for that I'm open to ideas.
https://github.com/jwilling/JNWCollectionView/pull/119#discussion_r15888348