Add center and size compound properties
Merge branch 'master' into compounds

Conflicts:
	Cartography.xcodeproj/project.pbxproj
	Cartography/LayoutProxy.swift
Make Expression parameter generic
Merge branch 'master' into compounds

Conflicts:
	Cartography/Property.swift
Give Expression multiple coefficients
Merge branch 'master' into compounds

Conflicts:
	Cartography/Expression.swift
Compound operators return NSLayoutConstraint[]
Merge branch 'master' into compounds
Add inequality operators for Compound
Add multiplication and division for Size
Add Edges
Add inset function for Edges
Update README
Overload ~ for compound attributes
Update README
Merge pull request #8 from robb/compounds

Add compound properties