Added LICENSE header to all files

src/node/byte_buffer.cc

src/node/byte_buffer.h

src/node/call.cc

src/node/call.h

src/node/channel.cc

src/node/channel.h

src/node/client.js

src/node/common.js

src/node/completion_queue_async_worker.cc

src/node/completion_queue_async_worker.h

src/node/credentials.cc

src/node/credentials.h

src/node/event.cc

src/node/event.h

src/node/examples/math_server.js

src/node/node_grpc.cc

src/node/port_picker.js

src/node/server.cc

src/node/server.h

src/node/server.js