Update to nan 2.0

src/node/binding.gyp

src/node/ext/byte_buffer.cc

src/node/ext/byte_buffer.h

src/node/ext/call.cc

src/node/ext/call.h

src/node/ext/channel.cc

src/node/ext/channel.h

src/node/ext/completion_queue_async_worker.cc

src/node/ext/completion_queue_async_worker.h

src/node/ext/credentials.cc

src/node/ext/credentials.h

src/node/ext/node_grpc.cc

src/node/ext/server.cc

src/node/ext/server.h

src/node/ext/server_credentials.cc

src/node/ext/server_credentials.h

src/node/package.json