Board - new methods and constants

* EMPTY is added as a constant of the empty cell;
* Initial state of the board is built according to the size of the
board;
* Methods for deciding whether the player can move and for skipping the
player's move;
Player's number of pieces refactor

It's smaller
Player class

The players are going to be placed in separate classes.
Constants in a file

Some constants were moved to an external file.