avocado.runner: New module with TestRunner class inside.

Creates the module `avocado.runner` where the `TestRunner` class lives.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
avocado.job: Remove TestRunner class.

Remove TestRunner class from this module, it's now defined inside new
module `avocado.runner`.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
avocado.plugins: Plugins vm and remote uses avocado.runner module.

Change plugins vm and remote to use new module `avocado.runner`
that contains the TestRunner class.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #294 from ruda/refactory_job_and_runner_modules

Move class TestRunner inside avocado.job into a proper new module avocado.runner