Remote machine plugin [V3]

Follow up of PR #245.

Introduce a new plugin that allows runs tests on a remote machine, by using the SSH connection provided by `avocado.utils.remote`. This plugins share many aspects of the vm plugin. Note that this plugins does not provides any form of provision, it depends on proper setup of the remote machine to execute.

Here's how to use:

    $ scripts/avocado run --remote-hostname 192.168.0.123 --remote-username ruda examples/tests/sleeptest.py examples/tests/failtest.py
    REMOTE LOGIN  : ruda@192.168.0.123:22
    JOB ID    : a32946182aba4bba484249fcac3c826f370c5453
    JOB LOG   : /home/rmoura/avocado/job-results/job-2014-10-23T11.45-a329461/job.log
    TESTS     : 2
    (1/2) examples/tests/sleeptest.py: PASS (1.00 s)
    (2/2) examples/tests/failtest.py: FAIL (0.00 s)
    PASS      : 1
    ERROR     : 0
    NOT FOUND : 0
    FAIL      : 1
    SKIP      : 0
    WARN      : 0
    TIME      : 1.01 s

Changes from V2 to V3:
* Rebase code.
* Include command line option to set SSH connection port (`--remote-port`) with default value to 22.
* Include manual page section "Running Remote Tests" to describe how to use this feature.
* Update RemoteMachinePlugin documentation: "...provided that you properly set it up by installing Avocado in it."