More control over formation changes.

Use player names in formation to have more control over what player moves to a new position.
The old way a single change in formation would cause a lot of movement and often leave the team vulnerable.

    def players
      {
        average: [:pepe, :andres, :luis, :juan, :jorge, :german],
        fast:    [:gaston, :pedro, :marcos],
        captain: [:nelson]
      }
    end

    def formation(match)
      formation.lineup = [
        [:pepe,  :none,  :andres, :none,   :none],
        [:luis,  :none,  :none,   :none,   :gaston],
        [:none,  :pedro, :none,   :nelson, :none],
        [:juan,  :none,  :none,   :none,   :marcos],
        [:jorge, :none,  :german, :none,   :none],
      ]
    end
Being honest, I didn't think about the opponent formation information when I proposed the names stuff :)

I think it's fair enough, in the context of this PR, to return just the players' type in the form of arrays.
We're planning to redefine opponent information provided to the coach, anyway (including players' actual position rather than static formation, for example).