message: add transaction cpp api and usage example
diff: CC=g++ -std=c++11
  #CC=ccache g++
  -all: POST-example GET-example Benchmark
  +all: POST-example GET-example Benchmark Transaction-example
  Benchmark: Benchmark.o
  - $(CC) -o Benchmark Benchmark.o -L../lib -lclient -lcurl -lpthread
  + $(CC) -o Benchmark Benchmark.o -L../lib -lclient -lpthread -lcurl
  GET-example: GET-example.o
  - $(CC) -o GET-example GET-example.o -I../ -lcurl -L../lib -lclient
  + $(CC) -o GET-example GET-example.o -I../ -L../lib -lclient -lcurl
  POST-example: POST-example.o
  - $(CC) -o POST-example POST-example.o -I../ -lcurl -L../lib -lclient
  + $(CC) -o POST-example POST-example.o -I../ -L../lib -lclient -lcurl
  +
  +Transaction-example:Transaction-example.o
  + $(CC) -o Transaction-example Transaction-example.o -I../ -L../lib -lclient -lcurl
  GET-example.o: GET-example.cpp ../client.cpp ../client.h
  $(CC) -c -I../ GET-example.cpp -o GET-example.o
  @@ -19,5 +22,8 @@ Benchmark.o: Benchmark.cpp ../client.cpp ../client.h
  POST-example.o: POST-example.cpp ../client.cpp ../client.h
  $(CC) -c -I../ POST-example.cpp -o POST-example.o
  +Transaction-example.o: Transaction-example.cpp ../client.cpp ../client.h
  + $(CC) -c -I../ Transaction-example.cpp -o Transaction-example.o
  +
  clean:
  - rm -rf *.o POST-example GET-example Benchmark
  + rm -rf *.o POST-example GET-example Benchmark Transaction-example
  