Use generated sitemap.xml instead of jekyll-sitemap gem

I copied the current sitemap.xml from [jekyll-sitemap](https://github.com/jekyll/jekyll-sitemap/blob/master/lib/sitemap.xml), because github pages is locking an old version of this gem, without the sitemap feature. More details [here](https://github.com/Helabs/helabs.github.com/issues/383#issuecomment-47357474)

You can see the new sitemap.xml in [staging](http://staging.helabs.com.br/sitemap.xml).

Closes #383 

//cc @mergulhao @thiagonzalez