Rewrite of keyword matching code. This fixes #20

Uses new iterator based tagged string functions.

src/data/card.hpp

@@ -56,7 +56,7 @@ public:
   IndexMap<FieldP, ValueP>& extraDataFor(const StyleSheet& stylesheet);
   
   /// Keyword usage statistics
-  vector<pair<Value*,const Keyword*>> keyword_usage;
+  vector<pair<const Value*,const Keyword*>> keyword_usage;
   
   /// Get the identification of this card, an identification is something like a name, title, etc.
   /** May return "" */