Columns

- Refactored column objects to contain a ::validate method, for the
  validation of incoming data.
- Context.columns is now a vector of one of each column object,
  indexed by attribute name, for validation purposes.

src/columns/ColUntil.h

@@ -35,6 +35,8 @@ class ColumnUntil : public ColumnDate
 public:
   ColumnUntil ();
   ~ColumnUntil ();
+
+  bool validate (std::string&);
 };
 
 #endif