Rename the Visual Studio >= 10 generators to indicate the version year:

 Visual Studio 10 => Visual Studio 10 2010
 Visual Studio 11 => Visual Studio 11 2012
 Visual Studio 12 => Visual Stduio 12 2013

Report the names with the year to the list of available generators so
that the cmake-gui drop-down shows the years.  When selecting a
generator from the "-G" option or from an existing CMAKE_GENERATOR cache
entry, recognize names without the years for compatibility and map them
to the names with years.

Update the generator names in the cmake-generators.7 manual.