avocado.utils.script: Introduce script module.

Introduce avocado.utils.script module to handle the creation
of scripts stored in the file system.

The module includes a flexible class for creating generic scripts and
 includes utilities functions too. Some examples:

    CONTENT = """
    #!/bin/sh
    echo "Hi There!"
    """

    x = avocado.utils.script.Script('/var/tmp/test.sh', CONTENT)
    x.save()
    ... do something with x, like run it ...
    x.remove()

    t = avocado.utils.script.make_temp_script('test.sh', CONTENT)
    ... do something with y, like run it ...

Avocado functional tests makes use of script creation,
so we will handle these creation using this proper module.

Signed-off-by: Rudá Moura <rmoura@redhat.com>