Add support for multiplier and constant values for vertical and horizontal pins

[jduff]

ex.

// anchor with constant
open func constraint(equalToConstant c: CGFloat) -> NSLayoutConstraint
open func constraint(greaterThanOrEqualToConstant c: CGFloat) -> NSLayoutConstraint
open func constraint(lessThanOrEqualToConstant c: CGFloat) -> NSLayoutConstraint

// anchor with multiplier
open func constraint(equalTo anchor: NSLayoutDimension, multiplier m: CGFloat) -> NSLayoutConstraint
open func constraint(greaterThanOrEqualTo anchor: NSLayoutDimension, multiplier m: CGFloat) -> NSLayoutConstraint
open func constraint(lessThanOrEqualTo anchor: NSLayoutDimension, multiplier m: CGFloat) -> NSLayoutConstraint

// anchor with multiplier and constant
open func constraint(equalTo anchor: NSLayoutDimension, multiplier m: CGFloat, constant c: CGFloat) -> NSLayoutConstraint
open func constraint(greaterThanOrEqualTo anchor: NSLayoutDimension, multiplier m: CGFloat, constant c: CGFloat) -> NSLayoutConstraint
open func constraint(lessThanOrEqualTo anchor: NSLayoutDimension, multiplier m: CGFloat, constant c: CGFloat) -> NSLayoutConstraint

• Pin horizontal and vertical anchors to constant value only dimensions can be pinned to constants
• Pin horizontal, vertical and dimension anchors with multiplier (only dimensions have multiplier)
• Pin horizontal, vertical and dimension anchors with multiplier and padding (only dimensions have multiplier)

[jduff]

fixed by #18

[jduff]

It looks like you can use multipliers with constraints other than dimensions. It is likely you can use constants with others as well. Should add support for those.