Conflicts:
	app/controllers/projects/services_controller.rb
	app/models/project_services/emails_on_push_service.rb

Supports four different event types all bundled under the "note" event type:

- comments on a commit
- comments on an issue
- comments on a merge request
- comments on a code snippet

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.

Conflicts:
	app/controllers/projects/services_controller.rb

Irker is a gateway which sends IRC messages on git updates. This new
service provides an interface to this gateway, integrated in Gitlab, for
each updates.
As per the guidelines, this commit adds the new feature in the
CHANGELOG, tests and documentation.

See http://www.catb.org/esr/irker/

See #1950

See #1809.

Make the following changes to deal with new behavior in Rails 4.1.2:

* Use nested resources to avoid slashes in arguments to path helpers.