Fix root subfolder parent (JohnSundell#75)

Return copied filed/folder when calling copy() (JohnSundell#91)

This patch fixes a regression that would cause a copied file or folder
not to be returned as part of that operation. A test has also been
added to make sure that this feature stays intact in the future.

Files 4.0 (JohnSundell#87)

Files 4.0

This change includes a brand new implementation for Files,
that keeps most of the public API intact, while modernizing
the underlying implementation.

Under the hood, Files now uses value types and protocols,
rather than class inheritance - which improves the type safety
of the API, and streamlines the underlying code. Error handling
is also improved to include more underlying info, and the docs
have been reworked to be much more thorough.

The API has also been fine-tuned and modernized, dropping the
`FileSystem` class in favor of more “Swifty” APIs on `Folder`,
introducing more options for creating new files and folders, and
making it possible to easily change properties on file system
sequences.

A list of all changes will be posted as part of this version’s
release notes.

Merge pull request JohnSundell#80 from JohnSundell/relative-paths

Add API for computing a path relative to a parent folder

README: Update for Swift 5.0

Merge pull request JohnSundell#77 from JohnSundell/remove-deprecated-api

Remove deprecated ‘creatingSubfolderFailed’ API

Merge pull request JohnSundell#53 from iainsmith/4.0

Add support for Swift 4.0 & update CI

Merge pull request JohnSundell#52 from krzyzanowskim/master

Set Swift 4.1. Remove Linux ObjCBool extension

Merge pull request JohnSundell#49 from ikhsan/master

Add append to file

Merge pull request JohnSundell#37 from JohnSundell/swift4-package-des…

…cription

Use Swift 4 version of the Package Description API