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
Create .travis.yml
Update README.md
Update README.md
Fixed issue where 'centerInView:' wasn't always using the correct superview
Merge pull request #24 from liamnichols/master

Fixed issue where 'centerInView:' wasn't always using the correct superview
Update README.md
updated demo project settings
cleaned up and documented demo project
added '-pinAttribute:toAttribute:ofItem:withConstant:relation:' method to match the soon to be deprecated pinEdge alternative
added seperate deprecated methods header file and updated deprecated tags to indicate what method to use instead
deprecated old methods that have been re-written better. deprecated warning instructs what method to use instead
added overview to depreacted header
Fully documented deprecated header
began to document the main header
moved deprecated methods back into main header (seperate interface). Removed usages of methods from demo vc, changed JRTViewPinEdges to NSUInteger so i can be included in appledoc (check that this is ok)
moved deprecated methods back into main header (seperate interface). Removed usages of methods from demo vc, changed JRTViewPinEdges to NSUInteger so i can be included in appledoc (check that this is ok)
added documentation for constrianing to size methods
fully documented header