Add support for Visual Studio 2013; the needed files are in contrib/vstudio/vc12/.
Additional changes in the new files, as compared to contrib/vstudio/vc11/:
- Drop obsolete files from projects:  infblock.h, infcodes.h, infutil.h.
  These missing files causes MSBuild to think that zlib always needs to
  be rebuilt.  Add missing header files to the projects.
- Remove the VERSION line in .def file.  The Visual Studio now enforces
  the syntax that VERSION can only be major[.minor], and does not allow
  a third number.  Anyway, the .def VERSION line is unnecessary, since
  the project has a Version resource.
- Rename the project that builds zlib as a .DLL from zlibvc to
  zlibwapi, so that the project name matches the .DLL name.
- In the zlibwapi (nee zlibvc) PreBuildEvent, fix the path to the
  directory with the batch file that builds assembler objects.  Add
  the PreBuildEvent to zlibstat, the project that builds zlib as a
  static library.