Escape Markdown characters and convert cross-issue links.

GitHub interprets several different characters as Markdown, and our
GoogleCode issue text is full of these chars.  This change transforms the
body (first comment) and later comments by escaping these Markdown chars.

Converts cross-issue links from GoogleCode format to GitHub format.
Add 5 second delay between each comment to reduce reorderings.
Without this delay, a full 1 in 3 comments migrated from GoogleCode to
GitHub is out of order with respect to other comments on the same issue!

A better and faster solution might be to query after each posted comment
but that would also eat into the rate limit.  This simple solution worked
well though.
Change default logging to INFO to give a much better idea of the progress
of the migration.