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/ColDate.h

@@ -40,11 +40,9 @@ public:
   ColumnDate ();
   ~ColumnDate ();
 
+  virtual bool validate (std::string&);
   virtual void measure (Task&, int&, int&);
   virtual void render (std::vector <std::string>&, Task&, int, Color&);
-
-protected:
-  std::string _attribute;
 };
 
 #endif