Make Cartfile optional if Cartfile.private exists
Document Cartfile.fromURL
Add loadCombinedCartfile
Refactor loadCombinedCartfile to return a signal
Add a regression test for loading a lone Cartfile.private
Use the beTruthy matcher instead
Make loadFromDirectory return a signal
Don't force unwrap
Zip cartfile signals
Merge pull request #256 from Carthage/only-private-files

Make Cartfile optional if Cartfile.private exists
Merge branch 'master' into carthage-fetch

Conflicts:
	Source/CarthageKit/Project.swift
	Source/carthage/Build.swift
	Source/carthage/Checkout.swift