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
Slight change to cookie-jar header

Instead of "Netscape HTTP Cookie File", now writing "HTTP Cookie File".
Python's cookie code recognizes either one.
Merge pull request #1 from mmorearty/patch-cookiejar-header

Add header line when writing to cookies.txt