Add cards 'Ravenous Rats' and 'Mind Rot'

This commit also adds helper functions for asking the player to choose
from a set of cards.
Attackers without Vigilance are now tapped.
Merge pull request #52 from gseitz/tap_attackers

Attackers without Vigilance are now tapped.
Describing ChoiceCard now uses describeObjectByRef
Merge remote-tracking branch 'origin/master'
Add reveal functionality + Lilian's Shade