gitlab-ce

Add notes notifications to services

This merge request builds on top of a few other merge requests:

* https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/237
* https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/350

The purpose of this merge request is to add service notification support when a user comments on one of the following:

* commits
* issues
* merge requests
* code snippets

Currently HipChat and Slack are only supported. The admin can select which events to toggle on or off.

## Questions

All note events are bundled under the `note` event type. Is this okay, or does it make more sense to have individual events and/or change the `object_type` attribute in the hook data?

Also, does it look better to link the entire `merge request #X` as opposed to just `#X`? I like the former.

## Before

![Screen Shot 2015-03-01 at 3.05.56 PM](https://gitlab.com/uploads/stanhu/gitlab-ce/8ae828e652/Screen_Shot_2015-03-01_at_3.05.56_PM.png)

## After

![Screen Shot 2015-03-06 at 6.24.40 AM](https://gitlab.com/uploads/gitlab-org/gitlab-ce/7b866ae02b/Screen_Shot_2015-03-06_at_6.24.40_AM.png)

### Slack

![Screen Shot 2015-03-01 at 2.59.00 PM](https://gitlab.com/uploads/stanhu/gitlab-ce/0147c2dc38/Screen_Shot_2015-03-01_at_2.59.00_PM.png)

### HipChat

![Screen Shot 2015-03-01 at 2.59.19 PM](https://gitlab.com/uploads/stanhu/gitlab-ce/14e78fc61c/Screen_Shot_2015-03-01_at_2.59.19_PM.png)

See merge request !358