Add tests on travis-ci.org

My script installs luarocks and then install luasocket using luarocks. I think luarocks good choice if we want move to some test freamework (busted, lunit e.t.c).
May be some one can write better rockspec and .travis.yml scripts but they works and i think this is good start. One thing. I do not know how determine windows version in rockspec. So i can not define `_WIN32_WINNT=0x0501` and `LUASOCKET_INET_PTON` to build correctly on WinXP/Win2k3.