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