Add tutorials
- Add Electronics/Experiment tutorial: Potentiometer
- Add NoSQL tutorial: Relational model
- Add NoSQL tutorial: NoSQL
- Add NoSQL tutorial: Column family (BigTable) stores
- Add NoSQL tutorial: Key-value stores
- Add Swift tutorial: Classes, part 2
- Add Swift tutorial: Classes, part 3
- Add Swift tutorial: Classes, part 4
- Add Swift tutorial: Classes, part 5
- Add Swift (macOS) tutorial: First application
- Add Swift (macOS) tutorial: Concurrency, (a)synchronicity and background processing
- Add Swift (macOS) tutorial: Basics of 2D graphics
- Add Swift (macOS) tutorial: Top main menu