Support nullptr on Visual Studio >=2005

Currently to support nullptr comparisons on Windows we have to add ``#define CATCH_CONFIG_CPP11_NULLPTR`` before including catch.hpp every time, as described in https://github.com/philsquared/Catch/issues/80 

But nullptr is certainly available since Visual Studio 2005, http://msdn.microsoft.com/en-us/library/4ex65770(v=vs.80).aspx   
 so  this can be done automatically.