auto_link should ignore multiple trailing punctuation marks

auto_link currently only ignores one trailing punctuation mark. This patch adds a test and support for an arbitrary number of trailing punctuations.

Previously

auto_link("Lorum ipsum sit dolor amet (http://example.com).")
=> "Lorum ipsum sit dolor amet (<a href=\"http://example.com)\">http://example.com)</a>."

With Patch

auto_link("Lorum ipsum sit dolor amet (http://example.com).")
=> "Lorum ipsum sit dolor amet (<a href=\"http://example.com\">http://example.com</a>)."

Note: I wrote this patch against the 2-3-stable branch.