Feature/005 define game mechanics and classes

[trsctr]

Issue: https://github.com/abbastoof/transcendence/issues/5

• [x] Implemented classes for Position, Paddle, Player, Ball and GameState

• [x] Defined attributes and methods, and implemented some of the methods for these entities

• [x] Wrote unit tests

• [x] Wrote documentation

• Some of possible methods are commented out, as implementing those is a separate issue: https://github.com/abbastoof/transcendence/issues/6

[mtoof]

Looks good