COMMON: Use true nullptr in Visual Studio 2010+ and true override in VS 2012+

Those compilers support these features despite not being fully C++11-compliant.

<http://msdn.microsoft.com/en-us/library/hh567368.aspx> says that VS 2010 has "partial" support for `override`. I don't know what that entails and I can't test it, so I err on the side of caution and only enable it in 2012 and up.