Add visual studio 2013 support

Thanks for @cyberium to point out my carelessness in https://github.com/cmangos/mangos-classic/pull/75.. I  closed it and resubmitted a pull request with all "ToolsVersion" fixed..

This pull request contains no technology at all, just hope to save time for who want to use visual studio 2013, and don't want to fight against git when commit something, thank you..

Also thanks to @Schmoozerd, @xfurry, @kid10 and all developers..

By the way, there are free visual studio 2013 downloads from microsoft official website:

Visual studio ultimate 2013 with update 2 90-day free trial:
http://go.microsoft.com/?linkid=9832426&clcid=0x804

Visual studio express 2013 with update 2 for windows:
http://go.microsoft.com/?linkid=9832246&clcid=0x804

Thank you..