Remote machine plugin [V4]

Follow up of PR #248.

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 V3 to V4:
* Split single commit in 3 commits (feature, tests, documentation).