Changed pointer to reference in Player::chooseCard

I see no reason why it should be a pointer.
Changed pointers to references in chooseCard calls

Now stuff compiles.
PerformBiddingTest changes

Tests now check whether the point multiplier is correct. Also removed some debug
code from Bidding.
Typo
Fixed issue #6

Arbiter was accepting doubles only from people from the same team, instead of
exactly the opposite.