add tgz support to tar creation keyword

[Nilsty]

I'd like to add the capabilities to create a compressed tar package (.tgz) with one keyword. I added an optional argument to the keyword create_tar_from_files_in_directory which will set the option "w:gz" to tarfile.open. I also added a test case example.

[lucagiove]

@Nilsty Unittest broken that show a not backward compatible change. I think this if is switched:

        if tgz:
            tar = tarfile.open(filename, "w")
        else:
            tar = tarfile.open(filename, "w:gz")

If confirmed please rebase and fix, thanks!