Fix reference error

Calling git init returns with an error, that self is not defined.

Example code:

```javascript
var git = require('git');
var path = require('path');
var repo = path.join(__dirname, String(Math.random() * 10000));

fs.mkdir(repo, function () {
    var g = new git.Git(repo);
    g.init({}, function (err, result) {
	console.log(err, result);
    });
});
```