Argument Parsing

- Implemented Arguments::extract_read_only_filter to isolate the
  arguments that pertain to read-only command filters
- Implemented Arguments::extract_write_filter to isolate the arguments
  that pertain to write command filters.
- Implemented Arguments::extract_modifications to isolate the arguments
  that pertain to write command modifications.
- Created stubbed Expression object.
- Began integration of Expression and Arguments into commands/CmdCustom.

src/Expression.h

@@ -0,0 +1,51 @@
+////////////////////////////////////////////////////////////////////////////////
+// taskwarrior - a command line task list manager.
+//
+// Copyright 2006 - 2011, Paul Beckingham, Federico Hernandez.
+// All rights reserved.
+//
+// This program is free software; you can redistribute it and/or modify it under
+// the terms of the GNU General Public License as published by the Free Software
+// Foundation; either version 2 of the License, or (at your option) any later
+// version.
+//
+// This program is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+// details.
+//
+// You should have received a copy of the GNU General Public License along with
+// this program; if not, write to the
+//
+//     Free Software Foundation, Inc.,
+//     51 Franklin Street, Fifth Floor,
+//     Boston, MA
+//     02110-1301
+//     USA
+//
+////////////////////////////////////////////////////////////////////////////////
+#ifndef INCLUDED_EXPRESSION
+#define INCLUDED_EXPRESSION
+#define L10N                                           // Localization complete.
+
+#include <string>
+#include <Arguments.h>
+#include <Task.h>
+
+class Expression
+{
+public:
+  Expression ();
+  Expression (Arguments&);
+  ~Expression ();
+  bool eval (Task&);
+
+  void dump (const std::string&);
+
+private:
+  void toInfix ();
+  void toPostfix ();
+};
+
+#endif
+////////////////////////////////////////////////////////////////////////////////