Issue 56 archivefile links

[kan-fu]

Fix #56

Added two methods (getArchivefileUrl and getArchivefileUrls) to only return the URLs of the archivefiles so users can use them in their favorite download manager.

Also added an example of how to do that in aria2c.

[spencerwplovie]

This is a nitpicky comment, but the 'else' is unnecessary, ie:

        if joinedWithNewline:
            return "\n".join(file_urls)
        return file_urls

[kan-fu]

Yeah. I normally omit the else when the else part is substantially bigger than the if part, so that it avoids the nesting for the longer code lines by ruling out those small and edge cases in the very beginning. For a one-liner, it looks more balanced to me inside an indentation block. Just a personal preference.