Merge swift4 into master

Release SwiftShell 3.0.

At long last.

Improve documentation for "run".

Also rename in LinuxMain.swift.

Update podspec with iOS deployment target 9.0.

Enable building for iOS. Just the Stream part. So FileSmith can use it.

The rest cannot build on iOS because it uses Process.
Move the Stream files into its own directory.

String(contentsOfFile:) not implemented on Linux, use String(contents…

…OfFile: encoding:) instead.

Make the default main.stdout a StdoutStream, which uses Swift.print().