Introduce avocado.utils.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 ...

Many Avocado's functional tests makes use of script creation, so we will handle these creation using this proper module.