Remote machine plugin [V2]

Follow up of PR #243 

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.

Here's an execution:

    $ 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
    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 the original PR:
* Accepted the suggestion of @clebergnu to rebase the commits, RemoteTest class and documentation plus tests, into a single commit for the feature.