@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.
Even if it's zero, are you able to specify the default value for this?