Added ability to specify path of .git folder

I haven't made an ember-cli extensions before so hopefully I didn't violate any conventions. These changes fix the issue for me. I can have my ember-project in a subfolder, specify the `.git` path and when I push the index it has the correct commit hash associated with it.
What are your thoughts on defaulting to the rootPath if no gitRoot is defined?  Something like:

```javascript
var rootPath       = this.project.root;
var gitRoot         = this.gitRoot || rootPath;
```