update supybot-botchk.sh
.gitignore: add plugins/ directory which is trying to get pushed into...

...this branch.
rename:    supybot-botchk.sh -> supybot-botchk-cron

supybot-botchk would have name conflict it would break this script
because of $PATH
supybot-botchk-cron-install
supybot-botchk-cron: update comments
supybot-botchk-cron-install: use double quotes.