Change forced bets: request small blind from the second, big blind from the third player
According to the rules, the player immediately clockwise from the dealer button (first player)
posts the "small blind" then the next player must post the "big blind"

Add some comments and new lines.
Calculate the special players depends from GameState#player_on_first_position
The first player has the dealer button.
The dealer button should move at the end of each round.
Also the game should keep on going while there is at least two players having a positive number of chips.

See the story card on trello: https://trello.com/c/GbqsAcqR
Renamed method missed from the previous commit.
Should be squashed with the previous commit.
The blinds should double when the dealer button returns to the first player.

See trello story card: https://trello.com/c/9drrg5er/44-the-blinds-should-double-when-the-dealer-button-returns-to-the-first-player
Merge remote-tracking branch 'origin/double_blinds'
Function rename
Refactor
Renamed player accessor functions on game_state to line up with standard poker terminology