Remove hardcoded symbols

[mattiadg]

In the code there are symbols like '.' in the board representation or magic numbers that should be changed into variable names.
This is good for newcomers.

[MuchtarSalimov]

May I work on this?

I plan to follow the style seen here and just add them to the header files:
const int BOARD_SIZE = 9;

[mattiadg]

Great, it's yours.

[mattiadg]

Addressed in #24 . Closing till the merging.