New method that allows specifying NSLayoutRelation when pinning edges

I'm using this functionality to solve the following example:

* I have a UIImageView constrained to 64x64 pinned to the left side of
my UIView
* I have a UILabel (titleLabel) pinned to the right of this UIImageView
* I have a second UILabel (descriptionLabel) pinned to the right of this
UIImageView

I want to have 10pts of margin between the main UIView's bottom edge,
and the edge of my UIImageView or descriptionLabel (whichever is
highest).

By using `NSLAyoutRelationLessThanOrEqual` I can pin both my UIImageView
_and_ my UILabel to the bottom of my parent UIView, and my UIView will
be appropriately sized.
Merge pull request #23 from bartvandendriessche/feature/allow_relation_specification

New method that allows specifying NSLayoutRelation when pinning edges