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/ColEntry.h

@@ -36,6 +36,7 @@ public:
   ColumnEntry ();
   ~ColumnEntry ();
 
+  bool validate (std::string&);
   void setStyle (const std::string&);
 };