Integration - Context::parse

- Integrated Att parsing.
- Unit tests are broken right now.
- Stubbed Context::shadow.
- Implemented Duration::valid.
- Task is broken right now.

src/Duration.h

@@ -42,6 +42,7 @@ public:
   operator int ();
   operator time_t ();
 
+  bool valid (const std::string&) const;
   void parse (const std::string&);
 
 private: