SSCollectionView - Add new "addRowSpacingToFirstRow" setting

The documentation for rowSpacing states "The spacing between each row in the receiver. This does not add space above the first row or below the last."  

First of all the code does seem to add spacing below the last already - `_rowSpacing` was being added to the height of every row - but for a project I was doing I needed the spacing before the first row as well.

This change adds a new property `addRowSpacingToFirstRow` which allows allows you to apply the rowSpacing before the first row of a section.  I've tested with and without scrolling Extremities and it seems to work.

The new setting defaults to `NO` which matches the previous behaviour.