add submodule 'khttp'

* https://github.com/kaija/khttp
add Makefile and initialize the folder architecture

.
├── Makefile
├── README.md
├── lib
│   └── khttp
├── src
│   ├── sm_api.c
│   └── sm_api.h
└── test
    ├── Makefile
    └── user_api.c