Added comments to node generation, also refactored some plugin code

examples/node/static_codegen/helloworld_grpc_pb.js

examples/node/static_codegen/helloworld_pb.js

...es/node/static_codegen/route_guide/route_guide_grpc_pb.js

examples/node/static_codegen/route_guide/route_guide_pb.js

src/compiler/cpp_generator_helpers.h

src/compiler/cpp_plugin.cc

src/compiler/csharp_generator.cc

src/compiler/csharp_generator_helpers.h

src/compiler/generator_helpers.h

src/compiler/node_generator.cc

src/compiler/node_generator.h

src/compiler/node_generator_helpers.h

src/compiler/node_plugin.cc

src/node/test/math/math_grpc_pb.js

src/node/test/math/math_pb.js