Twopence integration

Integration of Twopence and Pennyworth.

What is not ready:
 - installation instructions and installation script do not take into account the
    new dependency on twopence;
 - stdin cannot be redirected to a string;
 - inject_directory is still done with Cheetah.

What is already better:
 - one bug fixed: vm.run_command("ls -l") would not work,
   it was necessary to write vm.run_command("ls", "-l"), or to use the :as parameter;
 - we are directly calling libssh instead of spawning a bash to run a "ssh" command,
    so it's supposed to be faster;
 - twopence ruby extension is written in C, while Cheetah is apparently pure Ruby,
   so again it should be more efficient.

What could be better:
 - we don't use full power of Twopence, because we go only through SSH for now,
   we should take profit of virtio-serial, for example;
 - we could do pipes between commands, like Cheetah can do.