Updated to Swift 5.0, Removed Swift 3 support

Fixed podspec Swift version

Minor performance improvement on iCloud file progress monitor

Updated version, removed obsoleted properties

isReachable returns error (#87), Updating version

Updated project settings version to 0.22

Updated pod version to 0.21

Fixed currentPath docs, delegateNotify method default error arg

Refactored Dropbox, WebDAV & OneDrive to HTTPFileProvider

- FileOperationType.source is not optional anymore
- critical bugfix: KVO exception crash on task completion

Fixed typo