Change 'Labyrinth.cs' to 'Playfield.cs'

+ some private methods to public
Team work commit -  Bugfix resourse message; use properties insetad of fields; invalid command exception handling;
Created 'PlayfieldTest'

2 tests:
- StartRowTest;
- StartColTest;
Rename Unit Tests

- Cell.Tests;
- Playfield.Tests;
- ScoreBoard.Tests;