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.
Merge pull request #502 from SpecLad/visual-c++11

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