i18n

- Added en-US.h, which will be a header file full of string defines.
- Added logic to i18n.h to include en-US.h based on cmake command
  line argument (cmake ... -DPACKAGE_LANGUAGE=1).
- Added one sample string.

src/en-US.h

@@ -0,0 +1,47 @@
+////////////////////////////////////////////////////////////////////////////////
+// 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
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// Strings that should be localized:
+//   - All text output that the user sees or types
+//
+// Strings that should NOT be localized:
+//   - ./taskrc configuration variable names
+//   - certain literals associated with parsing
+//
+////////////////////////////////////////////////////////////////////////////////
+
+#ifndef INCLUDED_EN_US
+#define INCLUDED_EN_US
+
+// To localize, clone and rename this file, then change all the defines below.
+
+#define STRING_UNKNOWN_ERROR "Unknown error."
+
+#endif
+