File for Swift extensions and Operation class
Optional fallback operator
Revert back to NSOperationQueue extension

It was not possible build a layer above NSOperationQueue with generic NSOperations and so on. Extension approach will serve the best purpose.
Provide some default values for convenience init
Return back basic `perform` methods
Refactor default operation behavior
Return back delayed perform
Write more demo code
Update README with latest changes

Migrated from using GCD queues to NSOperationQueues, mostly.