Add header line when writing to cookies.txt

Note: This pull request directly parallels sparklemotion/mechanize#293.

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

See [this project](https://github.com/mmorearty/mechanize-cookieproblem) for a demo that uses Mechanize.  I didn't have a chance to write a demo that uses http-cookie.