Staging task

@fgrehm @aneziocampos @aluisioazevedo 

this PR will help to test the website on live GitHub environment (e.g. staging).
Running below task will create new branch named `staging`, push it to https://github.com/Helabs/staging.helabs.com.br and check out to the last branch where the work was going on.
```sh
$ rake staging
```
Staging version of the website will be available at [staging.helabs.com.br](http://staging.helabs.com.br).

Why we need it:
* With the introduction of jekyll plugins to Github Pages we have removed `safe: true` option from the [_config](https://github.com/Helabs/helabs.github.com/blob/master/_config.yml) file. So, there can happen the case when something works on local, but Github Pages doesn't support it yet. Problem is that we can only find it out after publishing it.
* Sometimes we need to test things with public URL. For example, testing AdWords, Content tag scores, PageSpeed (http://developers.google.com/speed/pagespeed/insights/) and etc.

For these, we need public staging version of the website where we can see things online.