Bug Fix - #299 Can't use multiple project.hasnt:foo

- Fixed bug that prevented a filter from containing multiple references
  to the same attribute (thanks for John Florian).
- Added unit tests to verify.

src/Context.h

@@ -70,7 +70,8 @@ public:
 private:
   void loadCorrectConfigFile ();
   void loadAliases ();
-  void autoFilter (Task&, Filter&);
+  void autoFilter (Att&, Filter&);
+  void autoFilter (Filter&);
 
 public:
   Config                    config;