fix #7, support C++ namespace

Usage: jsoncppgenerator.py [options] JSONFile

Options:
  -h, --help            show this help message and exit
  --dstdir=DSTDIR       directory to save the generated code files, default is
                        current directory
  --namespace=NAMESPACE
                        C++ namespace seperated with "::", for example,
                        "com::company". Default is no namespace
  --gentest             generate test code "main.cpp", default is false