Add set_breath and get_breath to lua API.

doc/lua_api.txt

src/client.cpp

src/client.h

src/clientserver.h

src/content_sao.cpp

src/content_sao.h

src/environment.cpp

src/environment.h

src/player.cpp

src/player.h

src/script/lua_api/l_object.cpp

src/script/lua_api/l_object.h

src/server.cpp

src/server.h