Add github_commit_info to webhook payload

Today I was using the webhook to post some information about the build into our chat channels. For example I'd like to show who was responsible for a failing build.

The commit id is a good start for this, but it requires talking to github which can be a pain and is actually not needed since strider already has all those information. 

This pull request adds a field "github_commit_info" to the webhook-payload if the build was triggered by a push that contains the whole job.github_commit_info.

This is an example of how the new payload looks like:

```
{
    "test_results": {
        "start_time": "2013-09-18T18: 21: 59.444Z",
        "repo_url": "https: //github.com/afd/asdf",
        "finish_time": "2013-09-18T18: 22: 04.953Z",
        "test_exitcode": 0,
        "deploy_exitcode": null,
        "github_commit_id": "7f0d739af1d36939de2615de71c0777e58b00553"
    },
    "github_commit_info": {
        "author": {
            "username": "asdf",
            "email": "asdf@asdf.com",
            "name": "asdf"
        },
        "timestamp": "2013-09-18T09: 45: 52-07: 00",
        "message": "Test",
        "id": "7f0d739af1d36939de2615de71c0777e58b00553"
    }
}
```