More information in merge request hook

**What does this MR do?**
This MR allow send more information about merge request via web hook: source project, target project and last commit.

Hook example:
```json
{
  "object_kind": "merge_request",
  "object_attributes": {
    "id": 99,
    "target_branch": "master",
    "source_branch": "ms-viewport",
    "source_project_id": 14,
    "author_id": 51,
    "assignee_id": 6,
    "title": "MS-Viewport",
    "created_at": "2013-12-03T17:23:34Z",
    "updated_at": "2013-12-03T17:23:34Z",
    "st_commits": null,
    "st_diffs": null,
    "milestone_id": null,
    "state": "opened",
    "merge_status": "unchecked",
    "target_project_id": 14,
    "iid": 1,
    "description": "",
    "source": {
      "name": "awesome_project",
      "ssh_url": "ssh://git@example.com/awesome_space/awesome_project.git",
      "http_url": "http://example.com/awesome_space/awesome_project.git",
      "visibility_level": 20,
      "namespace": "awesome_space"
    },
    "target": {
      "name": "awesome_project",
      "ssh_url": "ssh://git@example.com/awesome_space/awesome_project.git",
      "http_url": "http://example.com/awesome_space/awesome_project.git",
      "visibility_level": 20,
      "namespace": "awesome_space"
    },
    "last_commit": {
      "id": "da1560886d4f094c3e6c9ef40349f7d38b5d27d7",
      "message": "fixed readme",
      "timestamp": "2012-01-03T23:36:29+02:00",
      "url": "http://example.com/awesome_space/awesome_project/commits/da1560886d4f094c3e6c9ef40349f7d38b5d27d7",
      "author": {
        "name": "GitLab dev user",
        "email": "gitlabdev@dv6700.(none)"
      }
    }
  }
}
```

Was also affected by a piece of code responsible for the push event, as now has a certain common code base with Merga request

**Are there points in the code the reviewer needs to double check?**

- https://github.com/Bugagazavr/gitlabhq/blob/c3e05e3c7912440f64e65bbe9a5ae560c1193d35/app/models/merge_request.rb#L117-L121
- https://github.com/Bugagazavr/gitlabhq/blob/c3e05e3c7912440f64e65bbe9a5ae560c1193d35/app/services/git_push_service.rb#L154

**Why was this MR needed?**

- For many Continius Integration services, because now to get last commit sha, source remote ( if source is fork ) need work with API.
- GitLab doesn't have API to get last commits related with current PR, only project/branch.
- Also, I need this functionality to allow Drone to work with GitLab Merge Requests ( https://github.com/drone/drone/blob/exp/plugin/remote/gitlab/gitlab.go#L159-L162 )
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