Add support for PCB and footprint format versioning

Currently, kicad_pcb files have a (version ...) tag, but it is ignored. This
commit does the following:

1. Parse the version. If it's later than the last supported version, present an
alternative error message suggesting an upgrade if the file does not load
correctly. The version will be interpreted as an integer YYYYMMDD to suggest
a KiCad release date.

2. Accept a (version ...) tag also in kicad_mod files, but do not write them
yet. If no version tag is present in these files, assume the version to be that
of the current format version at the time of this commit.

This is meant to be merged to the 4.x stable series, and preps for KiCad 5
which will start emitting version tags also in footprints - users with what
will then be 'old stable' will not get a parse error when we introduce this for
footprints, and we can safely increment the format version later.

pcbnew/github/github_plugin.cpp

@@ -2,7 +2,7 @@
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
  * Copyright (C) 2015 SoftPLC Corporation, Dick Hollenbeck <dick@softplc.com>
- * Copyright (C) 2016 KiCad Developers, see CHANGELOG.TXT for contributors.
+ * Copyright (C) 2016 KiCad Developers, see AUTHORS.txt for contributors.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License