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;
 - an authorized SSH key has to be stored for non-root users, while this was
   not necessary previously, because Pennyworth was doing a su from root --
   however, I suggest we keep this incompatibility for orthogonality and security
   reasons.

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;
 - Twopence can store in one variable both stdout and stderr in the order in which they
   receive data, while Cheetah could not intermingle them and kept them separate;
 - 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;
 - the possibility to intermingle stdout and stderr is currently only accessible by
   calling Twopence directly -- add a new option to do that?
 - we could do pipes between commands, like Cheetah can do.

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