Makefile cleanup/additions.
Makefile cleanup/additions.

- Wrap comments to 80c (was a mix)
- Update some docs.
- Add test-all/test-everything that also invokes the Xcode tests.
- Add variables that can be set on the command line to tweak the xcodebuild
  invocation:
  - Enabling the analyzer.
  - Skipping the optimizer (to speed things up).

cburrows/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,allevato/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,tbkka/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf