Generate valid json
This patch helps to be compatible to other programing languages as it improves the validation of hook data. It seems only ruby can handle 'nil' as value while other json decode function will fatal.
See merge request !182
1) Adds a DB migration for all services to toggle on push, issue, and merge events.
2) Upon an issue or merge request event, fire service hooks.
3) Slack service supports custom messages for each of these events. Other services
not supported at the moment.
4) Label merge request hooks with their corresponding actions.
* add git_http_url and git_ssh_url to project web hook
* add visibility_level to project web hook
* add documentation about project visibility_level in API
