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/src/MTGDefinitions.cpp

@@ -137,7 +137,8 @@ const char* Constants::MTGBasicAbilities[] = {
     "mygraveexiler",
     "oppgraveexiler",
     "librarydeath",
-    "shufflelibrarydeath"
+    "shufflelibrarydeath",
+    "offering"
 };
 
 map<string,int> Constants::MTGBasicAbilitiesMap;