add option: ALLOW_PRIVATE_IP, set to yes by default.

this fix should remove this email report by cron:

Error: bad IP address: 0.0.0.0.

fix bug 177 save dig result in LAST_IP_FILE

- do not update when fatal error is not cleared by user.
- only update when IP is different from last successful update.
- send email to EMAIL and FATAL_ERROR_EMAIL

bugfix: do not "exit 0" after curl command.
bugfix: fix bug 175 use "cat" to get file content.

- added changelog;
- added bts;
- split config file from ddclient-http;
  example-config-file shipped with project;
- crontab gap increased to 4 minutes;

bugfix: nochg is changed from err to log. I think it's OK to get nochg right
after a update.

added doc in README.