Updated for Zabbix 2.2/2.4 ands new Slack API

#### In slack.sh
- Changed “RECOVERY” to “OK” in comments and scripts
- Changed to using the whole WebHook URL instead of just the token.
- Removed references to $1 and token

#### In README.md
- Added link to Zabbix 2.4 documentation
- Modified to reflect changes in script regarding webhook API
- Changed example format