Merge pull request #7 from robb/priority

Add ~ to set Priority
Update README
Add Baseline and FirstBaseline
Remove FirstBaseline since it's iOS 8 exclusive
Refactor edge tests
Refactor dimension tests
Refactor operator tests
Update README
Add division
Update README
Merge branch 'master' into compounds

Conflicts:
	Cartography.xcodeproj/project.pbxproj
	Cartography/LayoutProxy.swift