Added ping test

First step in end-to-end testing infrastructure. Enable with `ocaml setup.ml -configure --enable-end-test --enable-lwt`, rebuild and run with `sudo test/PingTest.py`. Starts up mininet with a single switch, and a controller that installs a repeater program, and runs a ping test and exits.
Merge pull request #58 from frenetic-lang/ping-test

Added ping test