SwiftPM Xcode project generation — merged

I have merged Xcode project generation into master.

To generate a project from a package:

    swift build --generate-xcodeproj

Or, the short-form:

    swift build -X

The above will work with the next available snapshot.

The project contains a dylib target per module and targets for any executables based on how it would build with SwiftPM normally.

There is an open list of TODO items in the Xcodeproj module directory: https://github.com/apple/swift-package-manager/tree/master/Sources/Xcodeproj