Add wren_math.c and wren_math.h to the MSVC 2013 project files, including both the .vcxproj and .vcxproj.filters. Also fix the indentation of the WREN_COMPUTED_GOTO comment in wren_common.h and correct a conditional guard bug in wren_vm.c where wrenLoadMathLibrary was guarded by WREN_USE_LIB_IO instead of WREN_USE_LIB_MATH.