Make the C++ generator use non-hardcoded namespaces.

Make the C++ generator use non-hardcoded namespaces.

- Make the C++ generator use program->get_namespace("cpp")
  instead of program->get_cpp_namespace()
- Eliminate the explicit "cpp_namespace" in t_program.
- Deprecate the cpp_namespace token.
- Update example .thrift files and syntax files.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665513 13f79535-47bb-0310-9956-ffa450edef68