Add header line when writing to cookies.txt

When Python is reading a Netscape-format cookies.txt file, it seems to require an initial line that looks like this:

    # Netscape HTTP Cookie File

Mechanize is not writing out a line like that, so Python scripts are unable to read cookie files that were created by Mechanize.  This patch adds that line.

See [this tiny project](https://github.com/mmorearty/mechanize-cookieproblem) for a testable demo.

By the way: I noticed that in #257, the cookie-handling part of Mechanize is in the process of being split off into a separate gem.  So I'll submit another similar pull request on the http-cookie project.