added basic offering support

usage tap lands to produce mana then sacrifice an offering... example
you have Guttersnipe in play, it has a mana cost of {2}{R}, and you have
Patron of the Akki in hand that costs {4}{R}{R}. You need to produce
{2}{R} mana first before casting Patron of the Akki, then choose
Guttersnipe as an offering.

projects/mtg/include/ExtraCost.h

@@ -254,6 +254,17 @@ public:
   virtual Ninja * clone() const;
 };
 
+//offering cost
+class Offering : public ExtraCost
+{
+public:
+  Offering(TargetChooser *_tc = NULL);
+  virtual int canPay();
+  virtual int isPaymentSet();
+  virtual int doPay();
+  virtual Offering * clone() const;
+};
+
 class CounterCost : public ExtraCost
 {
 public: